Linux 2.4 on:
[Wikipedia]
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The Linux kernel is a
free and open-source
Free and open-source software (FOSS) is a term used to refer to groups of software consisting of both free software and open-source software where anyone is freely licensed to use, copy, study, and change the software in any way, and the source ...
, monolithic
A monolith is a monument or natural feature consisting of a single massive stone or rock.
Monolith or monolithic may also refer to:
Architecture
* Monolithic architecture, a style of construction in which a building is carved, cast or excavated ...
, modular
Broadly speaking, modularity is the degree to which a system's components may be separated and recombined, often with the benefit of flexibility and variety in use. The concept of modularity is used primarily to reduce complexity by breaking a s ...
, multitasking, Unix-like
A Unix-like (sometimes referred to as UN*X or *nix) operating system is one that behaves in a manner similar to a Unix system, although not necessarily conforming to or being certified to any version of the Single UNIX Specification. A Unix-li ...
operating system kernel
Kernel may refer to:
Computing
* Kernel (operating system), the central component of most operating systems
* Kernel (image processing), a matrix used for image convolution
* Compute kernel, in GPGPU programming
* Kernel method, in machine lea ...
. It was originally authored in 1991 by Linus Torvalds
Linus Benedict Torvalds ( , ; born 28 December 1969) is a Finnish software engineer who is the creator and, historically, the lead developer of the Linux kernel, used by Linux distributions and other operating systems such as Android. He also ...
for his i386
The Intel 386, originally released as 80386 and later renamed i386, is a 32-bit microprocessor introduced in 1985. The first versions had 275,000 transistorsGNU operating system, which was written to be a free (libre) replacement for
In April 1991, 
On 19 January 1992, the first post to the new newsgroup ''alt.os.linux'' was submitted. On 31 March 1992, the newsgroup was renamed ''comp.os.linux''. The fact that Linux is a
Unix
Unix (; trademarked as UNIX) is a family of multitasking, multiuser computer operating systems that derive from the original AT&T Unix, whose development started in 1969 at the Bell Labs research center by Ken Thompson, Dennis Ritchie, a ...
.
Linux is provided under the GNU General Public License version 2 only, but it contains files under other compatible licenses.
Since the late 1990s, it has been included as part of a large number of operating system distributions, many of which are commonly also called Linux
Linux ( or ) is a family of open-source Unix-like operating systems based on the Linux kernel, an operating system kernel first released on September 17, 1991, by Linus Torvalds. Linux is typically packaged as a Linux distribution, which i ...
.
Linux is deployed on a wide variety of computing systems, such as embedded device
An embedded system is a computer system—a combination of a computer processor, computer memory, and input/output peripheral devices—that has a dedicated function within a larger mechanical or electronic system. It is ''embedded'' a ...
s, mobile device
A mobile device (or handheld computer) is a computer small enough to hold and operate in the hand. Mobile devices typically have a flat LCD or OLED screen, a touchscreen interface, and digital or physical buttons. They may also have a physical ...
s (including its use in the Android
Android may refer to:
Science and technology
* Android (robot), a humanoid robot or synthetic organism designed to imitate a human
* Android (operating system), Google's mobile operating system
** Bugdroid, a Google mascot sometimes referred to ...
operating system), personal computer
A personal computer (PC) is a multi-purpose microcomputer whose size, capabilities, and price make it feasible for individual use. Personal computers are intended to be operated directly by an end user, rather than by a computer expert or tech ...
s, servers, mainframes
A mainframe computer, informally called a mainframe or big iron, is a computer used primarily by large organizations for critical applications like bulk data processing for tasks such as censuses, industry and consumer statistics, enterpris ...
, and supercomputers. It can be tailored for specific architectures and for several usage scenarios using a family of simple commands (that is, without the need of manually editing its source code before compilation); privileged users can also fine-tune kernel parameters at runtime. Most of the Linux kernel code is written using the GNU extensions of GCC to the standard C programming language
''The C Programming Language'' (sometimes termed ''K&R'', after its authors' initials) is a computer programming book written by Brian Kernighan and Dennis Ritchie, the latter of whom originally designed and implemented the language, as well as ...
and with the use of architecture-specific instructions (ISA
Isa or ISA may refer to:
Places
* Isa, Amur Oblast, Russia
* Isa, Kagoshima, Japan
* Isa, Nigeria
* Isa District, Kagoshima, former district in Japan
* Isa Town, middle class town located in Bahrain
* Mount Isa, Queensland, Australia
* Mount Is ...
) in limited parts of the kernel. This produces a highly optimized executable ( vmlinux) with respect to utilization of memory space and task execution times.
Day-to-day development discussions take place on the Linux kernel mailing list (LKML). Changes are tracked using the version control system git, which was originally authored by Torvalds as a free software replacement for BitKeeper
BitKeeper is a software tool for distributed revision control of computer source code. Originally developed as proprietary software by BitMover Inc., a privately held company based in Los Gatos, California, it was released as open-source software ...
.
History
In April 1991, Linus Torvalds
Linus Benedict Torvalds ( , ; born 28 December 1969) is a Finnish software engineer who is the creator and, historically, the lead developer of the Linux kernel, used by Linux distributions and other operating systems such as Android. He also ...
, at the time a 21-year-old computer science
Computer science is the study of computation, automation, and information. Computer science spans theoretical disciplines (such as algorithms, theory of computation, information theory, and automation) to practical disciplines (includin ...
student at the University of Helsinki
The University of Helsinki ( fi, Helsingin yliopisto, sv, Helsingfors universitet, abbreviated UH) is a public research university located in Helsinki, Finland since 1829, but founded in the city of Turku (in Swedish ''Åbo'') in 1640 as the ...
, Finland
Finland ( fi, Suomi ; sv, Finland ), officially the Republic of Finland (; ), is a Nordic country in Northern Europe. It shares land borders with Sweden to the northwest, Norway to the north, and Russia to the east, with the Gulf of Bo ...
, started working on some simple ideas for an operating system inspired by UNIX, for a personal computer. He started with a task switcher
is the common name for a keyboard shortcut that has been in Microsoft Windows since Windows 2.0 (1987). This shortcut switches between application-level Window (computing), windows without using the mouse; hence it was named ''Task Switcher'' (' ...
in Intel 80386 assembly language and a terminal driver. On 25 August 1991, Torvalds posted the following to ''comp.os.minix'', a newsgroup
A Usenet newsgroup is a repository usually within the Usenet system, for messages posted from users in different locations using the Internet. They are discussion groups and are not devoted to publishing news. Newsgroups are technically distin ...
on Usenet
Usenet () is a worldwide distributed discussion system available on computers. It was developed from the general-purpose Unix-to-Unix Copy (UUCP) dial-up network architecture. Tom Truscott and Jim Ellis conceived the idea in 1979, and it was ...
:
On 17 September 1991, Torvalds prepared version 0.01 of Linux and put on the "ftp.funet.fi" – FTP server of the Finnish University and Research Network ( FUNET). It was not even executable since its code still needed Minix for compilation and play.
On 5 October 1991, Torvalds announced the first "official" version of Linux, version 0.02. At this point, Linux was able to run Bash, GCC, and some other GNU utilities:
After that, despite the limited functionality of the early versions, Linux rapidly gained developers and users. Many people contributed code to the project, including some developers from the MINIX community. At the time, the GNU Project
The GNU Project () is a free software, mass collaboration project announced by Richard Stallman on September 27, 1983. Its goal is to give computer users freedom and control in their use of their computers and Computer hardware, computing devi ...
had created many of the components required for its free UNIX replacement, the GNU operating system, but its own kernel, GNU Hurd
GNU Hurd is a collection of microkernel servers written as part of GNU, for the GNU Mach microkernel. It has been under development since 1990 by the GNU Project of the Free Software Foundation, designed as a replacement for the Unix kernel, ...
, was incomplete. For this reason, it soon adopted the Linux kernel as well. The Berkeley Software Distribution
The Berkeley Software Distribution or Berkeley Standard Distribution (BSD) is a discontinued operating system based on Research Unix, developed and distributed by the Computer Systems Research Group (CSRG) at the University of California, Ber ...
had not yet freed itself from legal encumbrances and was not competing in the space for a free OS kernel.
Torvalds assigned version 0 to the kernel to indicate that it was mainly for testing and not intended for productive use. Version 0.11, released in December 1991, was the first self-hosted Linux, for it could be compiled by a computer running the same kernel.
When Torvalds released version 0.12 in February 1992, he adopted the GNU General Public License
The GNU General Public License (GNU GPL or simply GPL) is a series of widely used free software licenses that guarantee end user
In product development, an end user (sometimes end-user) is a person who ultimately uses or is intended to ulti ...
version 2 (GPLv2) over his previous self-drafted license, which had not permitted commercial redistribution. In contrast to Unix
Unix (; trademarked as UNIX) is a family of multitasking, multiuser computer operating systems that derive from the original AT&T Unix, whose development started in 1969 at the Bell Labs research center by Ken Thompson, Dennis Ritchie, a ...
, all source files of Linux are freely available, including device drivers
In computing, a device driver is a computer program that operates or controls a particular type of device that is attached to a computer or automaton. A driver provides a software interface to hardware devices, enabling operating systems and o ...
. The initial success of Linux was driven by programmers and testers across the world. With the support of the POSIX
The Portable Operating System Interface (POSIX) is a family of standards specified by the IEEE Computer Society for maintaining compatibility between operating systems. POSIX defines both the system- and user-level application programming inte ...
APIs, through the libC that, whether needed, acts as an entry point to the kernel address space, Linux could run software and applications that had been developed for Unix.
On 19 January 1992, the first post to the new newsgroup ''alt.os.linux'' was submitted. On 31 March 1992, the newsgroup was renamed ''comp.os.linux''. The fact that Linux is a monolithic kernel
A monolithic kernel is an operating system architecture where the entire operating system is working in kernel space. The monolithic model differs from other operating system architectures (such as the microkernel architecture) in that it alone d ...
rather than a microkernel
In computer science, a microkernel (often abbreviated as μ-kernel) is the near-minimum amount of software that can provide the mechanisms needed to implement an operating system (OS). These mechanisms include low-level address space management, ...
was the topic of a debate between Andrew S. Tanenbaum, the creator of MINIX, and Torvalds. The Tanenbaum–Torvalds debate
The Tanenbaum–Torvalds debate was a written debate between Andrew S. Tanenbaum and Linus Torvalds, regarding the Linux kernel and kernel architecture in general. Tanenbaum, the creator of Minix, began the debate in 1992 on the Usenet discus ...
started in 1992 on the Usenet
Usenet () is a worldwide distributed discussion system available on computers. It was developed from the general-purpose Unix-to-Unix Copy (UUCP) dial-up network architecture. Tom Truscott and Jim Ellis conceived the idea in 1979, and it was ...
group ''comp.os.minix'' as a general discussion about kernel architectures.
Linux version 0.95 was the first to be capable of running the X Window System
The X Window System (X11, or simply X) is a windowing system for bitmap displays, common on Unix-like operating systems.
X provides the basic framework for a GUI environment: drawing and moving windows on the display device and interacting ...
. In March 1994, Linux 1.0.0 was released with 176,250 lines of code. It was the first version suitable for use in production environments.
It started a versioning system for the kernel with three or four numbers separated by dots where the first represented the ''major'' release, the second was the ''minor release'', and the third was the ''revision.'' At that time odd-numbered ''minor'' releases were for development and tests, whilst even numbered ''minor'' releases were for production. The optional fourth digit indicated a set of patches to a ''revision.'' Development releases were indicated with ''-rc'' ("release candidate") suffix.
The current version numbering is slightly different from the above. The even vs. odd numbering has been dropped, and a specific ''major'' version is now indicated by the first two numbers, taken as a whole. While the time-frame is open for the development of the next ''major'', the -rcN suffix is used to identify the n'th ''release candidate'' for the next version. For example, the release of the version 4.16 was preceded by seven 4.16-rcN (from -rc1 to -rc7). Once a stable release is made, its maintenance is passed off to the "stable team". Occasional updates to stable releases are identified by a three numbering scheme (e.g., 4.13.1, 4.13.2, ..., 4.13.16).
After version 1.3 of the kernel, Torvalds decided that Linux had evolved enough to warrant a new ''major'' number, so he released version 2.0.0 in June 1996. The series included 41 releases. The major feature of 2.0 was support for symmetric multiprocessing
Symmetric multiprocessing or shared-memory multiprocessing (SMP) involves a multiprocessor computer hardware and software architecture where two or more identical processors are connected to a single, shared main memory, have full access to all ...
(SMP) and support for more types of processors.
Starting with version 2.0, Linux is configurable for selecting specific hardware targets and for enabling architecture-specific features and optimizations. The ''make *config'' family of commands of ''kbuild'' are used to enable and configure thousands of options for building ad hoc kernel executables ( vmlinux) and loadable modules.
Version 2.2, released on 20 January 1999, improved locking granularity and SMP management, added m68k
The Motorola 68000 series (also known as 680x0, m68000, m68k, or 68k) is a family of 32-bit complex instruction set computer (CISC) microprocessors. During the 1980s and early 1990s, they were popular in personal computers and workstations and ...
, PowerPC
PowerPC (with the backronym Performance Optimization With Enhanced RISC – Performance Computing, sometimes abbreviated as PPC) is a reduced instruction set computer (RISC) instruction set architecture (ISA) created by the 1991 Apple– IBM ...
, Sparc64, Alpha, and other 64-bit platforms support. Furthermore, it added new file systems
In computing, file system or filesystem (often abbreviated to fs) is a method and data structure that the operating system uses to control how data is stored and retrieved. Without a file system, data placed in a storage medium would be one larg ...
including Microsoft
Microsoft Corporation is an American multinational corporation, multinational technology company, technology corporation producing Software, computer software, consumer electronics, personal computers, and related services headquartered at th ...
's NTFS
New Technology File System (NTFS) is a proprietary journaling file system developed by Microsoft. Starting with Windows NT 3.1, it is the default file system of the Windows NT family. It superseded File Allocation Table (FAT) as the preferred f ...
read-only capability. In 1999, IBM published its patches to the Linux 2.2.13 code for the support of the S/390 architecture.
Version 2.4.0, released on 4 January 2001, contained support for ISA
Isa or ISA may refer to:
Places
* Isa, Amur Oblast, Russia
* Isa, Kagoshima, Japan
* Isa, Nigeria
* Isa District, Kagoshima, former district in Japan
* Isa Town, middle class town located in Bahrain
* Mount Isa, Queensland, Australia
* Mount Is ...
Plug and Play
In computing, a plug and play (PnP) device or computer bus is one with a specification that facilitates the recognition of a hardware component in a system without the need for physical device configuration or user intervention in resolving resou ...
, USB, and PC Card
In computing, PC Card is a configuration for computer parallel communication peripheral interface, designed for laptop computers. Originally introduced as PCMCIA, the PC Card standard as well as its successors like CardBus were defined and devel ...
s. Linux 2.4 added support for the Pentium 4
Pentium 4 is a series of single-core CPUs for desktops, laptops and entry-level servers manufactured by Intel. The processors were shipped from November 20, 2000 until August 8, 2008. The production of Netburst processors was active from 2000 ...
and Itanium
Itanium ( ) is a discontinued family of 64-bit Intel microprocessors that implement the Intel Itanium architecture (formerly called IA-64). Launched in June 2001, Intel marketed the processors for enterprise servers and high-performance comp ...
(the latter introduced the ia64
IA-64 (Intel Itanium architecture) is the instruction set architecture (ISA) of the Itanium family of 64-bit Intel microprocessors. The basic ISA specification originated at Hewlett-Packard (HP), and was subsequently implemented by Intel in co ...
ISA that was jointly developed by Intel and Hewlett-Packard to supersede the older PA-RISC
PA-RISC is an instruction set architecture (ISA) developed by Hewlett-Packard. As the name implies, it is a reduced instruction set computer (RISC) architecture, where the PA stands for Precision Architecture. The design is also referred to a ...
), and for the newer 64-bit MIPS processor. Development for 2.4.''x'' changed a bit in that more features were made available throughout the duration of the series, including support for Bluetooth
Bluetooth is a short-range wireless technology standard that is used for exchanging data between fixed and mobile devices over short distances and building personal area networks (PANs). In the most widely used mode, transmission power is limit ...
, Logical Volume Manager (LVM) version 1, RAID
Raid, RAID or Raids may refer to:
Attack
* Raid (military), a sudden attack behind the enemy's lines without the intention of holding ground
* Corporate raid, a type of hostile takeover in business
* Panty raid, a prankish raid by male colleg ...
support, InterMezzo
In music, an intermezzo (, , plural form: intermezzi), in the most general sense, is a composition which fits between other musical or dramatic entities, such as acts of a play or movements of a larger musical work. In music history, the term ha ...
and ext3
ext3, or third extended filesystem, is a journaled file system that is commonly used by the Linux kernel. It used to be the default file system for many popular Linux distributions. Stephen Tweedie first revealed that he was working on exten ...
file systems.
Version 2.6.0 was released on 17 December 2003. The development for 2.6.''x'' changed further towards including new features throughout the duration of the series. Among the changes that have been made in the 2.6 series are: integration of µClinux into the mainline kernel sources, PAE support, support for several new lines of CPUs, integration of Advanced Linux Sound Architecture (ALSA) into the mainline kernel sources, support for up to 232 users (up from 216), support for up to 229 process IDs (64-bit only, 32-bit arches still limited to 215), substantially increased the number of device types and the number of devices of each type, improved 64-bit
In computer architecture, 64-bit integers, memory addresses, or other data units are those that are 64 bits wide. Also, 64-bit CPUs and ALUs are those that are based on processor registers, address buses, or data buses of that size. A comp ...
support, support for file system
In computing, file system or filesystem (often abbreviated to fs) is a method and data structure that the operating system uses to control how data is stored and retrieved. Without a file system, data placed in a storage medium would be one lar ...
s which support file sizes of up to 16 terabyte
The byte is a unit of digital information that most commonly consists of eight bits. Historically, the byte was the number of bits used to encode a single character of text in a computer and for this reason it is the smallest addressable unit ...
s, in-kernel preemption, support for the Native POSIX Thread Library (NPTL), User-mode Linux integration into the mainline kernel sources, SELinux
Security-Enhanced Linux (SELinux) is a Linux kernel security module that provides a mechanism for supporting access control security policies, including mandatory access controls (MAC).
SELinux is a set of kernel modifications and user-space ...
integration into the mainline kernel sources, InfiniBand
InfiniBand (IB) is a computer networking communications standard used in high-performance computing that features very high throughput and very low latency. It is used for data interconnect both among and within computers. InfiniBand is also us ...
support, and considerably more.
Also notable are the addition of a wide selection of file systems starting with the 2.6.''x'' releases: now, the kernel supports a large number of file systems, some that have been designed for Linux, like ext3
ext3, or third extended filesystem, is a journaled file system that is commonly used by the Linux kernel. It used to be the default file system for many popular Linux distributions. Stephen Tweedie first revealed that he was working on exten ...
, ext4
ext4 (fourth extended filesystem) is a journaling file system for Linux, developed as the successor to ext3.
ext4 was initially a series of backward-compatible extensions to ext3, many of them originally developed by Cluster File Systems fo ...
, FUSE
Fuse or FUSE may refer to:
Devices
* Fuse (electrical), a device used in electrical systems to protect against excessive current
** Fuse (automotive), a class of fuses for vehicles
* Fuse (hydraulic), a device used in hydraulic systems to prote ...
, Btrfs
Btrfs (pronounced as "better F S", "butter F S", "b-tree F S", or simply by spelling it out) is a computer storage format that combines a file system based on the copy-on-write (COW) principle with a logical volume manager (not to be confused ...
, and others that are native of other operating systems like JFS, XFS, Minix, Xenix, Irix
IRIX ( ) is a discontinued operating system developed by Silicon Graphics (SGI) to run on the company's proprietary MIPS workstations and servers. It is based on UNIX System V with BSD extensions. In IRIX, SGI originated the XFS file system ...
, Solaris, System V
Unix System V (pronounced: "System Five") is one of the first commercial versions of the Unix operating system. It was originally developed by AT&T and first released in 1983. Four major versions of System V were released, numbered 1, 2, 3, an ...
, Windows
Windows is a group of several proprietary graphical operating system families developed and marketed by Microsoft. Each family caters to a certain sector of the computing industry. For example, Windows NT for consumers, Windows Server for ...
and MS-DOS
MS-DOS ( ; acronym for Microsoft Disk Operating System, also known as Microsoft DOS) is an operating system for x86-based personal computers mostly developed by Microsoft. Collectively, MS-DOS, its rebranding as IBM PC DOS, and a few oper ...
.
In 2005 the ''stable team'' was formed as a response to the lack of a kernel tree where people could work on bug fixes, and it would keep updating ''stable'' versions. In February 2008 the ''linux-next'' tree was created to serve as a place where patches aimed to be merged during the next development cycle gathered. Several subsystem maintainers also adopted the suffix ''-next'' for trees containing code which they mean to submit for inclusion in the next release cycle. , the in-development version of Linux is held in an unstable branch named ''linux-next''.
Linux used to be maintained without the help of an automated source code management system until, in 2002, development switched to BitKeeper
BitKeeper is a software tool for distributed revision control of computer source code. Originally developed as proprietary software by BitMover Inc., a privately held company based in Los Gatos, California, it was released as open-source software ...
. It was freely available for Linux developers but it was not free software
Free software or libre software is computer software distributed under terms that allow users to run the software for any purpose as well as to study, change, and distribute it and any adapted versions. Free software is a matter of liberty, ...
. In 2005, because of efforts to reverse-engineer it, the company which owned the software revoked its support of the Linux community. In response, Torvalds and others wrote Git. The new system was written within weeks, and in two months the first official kernel made using it was released.
Details on the history of the 2.6 kernel series can be found in the ChangeLog files on the 2.6 kernel series source code release area of kernel.org.
The 20th anniversary of Linux was celebrated by Torvalds in July 2011 with the release of the 3.0.0 kernel version. As 2.6 had been the version number for 8 years, a new ''uname26'' personality that reports 3.x as 2.6.40+x had to be added to the kernel so that old programs would work.
Version 3.0 was released on 22 July 2011. On 30 May 2011, Torvalds announced that the big change was "NOTHING. Absolutely nothing." and asked, "...let's make sure we really make the next release not just an all new shiny number, but a good kernel too." After the expected 6–7 weeks of the development process, it would be released near the 20th anniversary of Linux.
On 11 December 2012, Torvalds decided to reduce kernel complexity by removing support for i386
The Intel 386, originally released as 80386 and later renamed i386, is a 32-bit microprocessor introduced in 1985. The first versions had 275,000 transistors The same series unified support for the ARM processor.
Version 3.11, released on 2 September 2013, adds many new features such as new flag for to reduce temporary file vulnerabilities, experimental AMD
Linux is a
Linux is a clone of UNIX, and aims toward
There are several kernel internal APIs utilized between the different subsystems. Some are available only within the kernel subsystems, while a somewhat limited set of in-kernel symbols (i.e., variables, data structures, and functions) is exposed also to dynamically loadable modules (e.g., device drivers loaded on demand) whether they're exported with the and macros (the latter reserved to modules released under a GPL-compatible license).
Linux provides in-kernel APIs that manipulate data structures (e.g.,
Kernel includes SLAB, SLUB and SLOB allocators as configurable alternatives. SLUB is the newest and it is also the default allocator. It aims for simplicity and efficiency. SLUB has been made
While not originally designed to be portable, Linux is now one of the most widely ported operating system kernels, running on a diverse range of systems from the
ClangBuiltLinux
' group coordinates fixes to both
Bugs involving the Linux Kernel can be difficult to troubleshoot. This is because of the kernel's interaction with userspace and hardware; and also because they might be caused from a wider range of reasons compared to those of user programs. A few examples of the underlying causes are semantic errors in code, misuse of synchronization primitives, and incorrect hardware management.
A report of a non-fatal bug in the kernel is called an " oops"; such deviations from correct behavior of the Linux kernel may allow continued operation with compromised reliability.
A critical and fatal error is reported via the function. It prints a message and then halts the kernel.
One of the most common techniques used to find out bugs in code is ''debugging by printing''. For this purpose Linux provides an in-kernel API called which stores messages in a circular buffer. The system call is used for reading and/or clearing the kernel message ring buffer and for setting the maximum ''log level'' of the messages to be sent to the console (i.e., one of the eight parameters of , which tell the severity of the condition reported); usually it is invoked via the glibC wrapper . Kernel messages are also exported to userland through the ''/dev/kmsg'' interface (e.g., systemd-journald reads that interface and by default append the messages to ).
Another fundamental technique for debugging a running kernel is tracing. The '' ftrace'' mechanism is a Linux internal tracer; it is used for monitoring and debugging Linux at runtime and it can also analyze user space latencies due to kernel misbehavior. Furthermore, ''ftrace'' allows users to trace Linux at boot-time.
''kprobes'' and ''kretprobes'' can break (like debuggers in userspace) into Linux and non-disruptively collect information. ''kprobes'' can be inserted into code at (almost) any address, while kretprobes work at function return. ''uprobes'' have similar purposes but they also have some differences in usage and implementation.
With KGDB Linux can be debugged in much the same way as userspace programs. KGDB requires an additional machine that runs GDB and that is connected to the target to be debugged using a serial cable or
There are certain communities that develop kernels based on the official Linux. Some interesting bits of code from these forks that include
The cost to redevelop version 2.6.0 of the Linux kernel in a traditional proprietary development setting has been estimated to be US$612 million (€467M, £394M) in 2004 prices using the COCOMO person-month estimation model. In 2006, a study funded by the European Union put the redevelopment cost of kernel version 2.6.8 higher, at €882M ($1.14bn, £744M).
This topic was revisited in October 2008 by Amanda McPherson, Brian Proffitt, and Ron Hale-Evans. Using David A. Wheeler's methodology, they estimated redevelopment of the 2.6.25 kernel now costs $1.3bn (part of a total $10.8bn to redevelop Fedora 9). Again, Garcia-Garcia and Alonso de Magdaleno from University of Oviedo (Spain) estimate that the value annually added to kernel was about €100M between 2005 and 2007 and €225M in 2008, it would cost also more than €1bn (about $1.4bn as of February 2010) to develop in the European Union.
, using then-current LOC (lines of code) of a 2.6.x Linux kernel and wage numbers with David A. Wheeler's calculations it would cost approximately $3bn (about €2.2bn) to redevelop the Linux kernel as it keeps getting bigger. An updated calculation , using then-current 20,088,609 LOC (lines of code) for the 4.14.14 Linux kernel and the current US national average programmer salary of $75,506 show it would cost approximately $14,725,449,000 dollars (£11,191,341,000) to rewrite the existing code.
The latest kernel version and older kernel versions are maintained separately. Most latest kernel releases were supervised by Linus Torvalds.
The Linux kernel developer community maintains a stable kernel by applying fixes for software bugs that have been discovered during the development of the subsequent stable kernel. Therefore, www.kernel.org will always list two stable kernels. The next stable Linux kernel is now released only 8 to 12 weeks later. Therefore, the Linux kernel maintainers have designated some stable kernel releases as ''longterm'', these
Kernel bugzilla
an
regressions
for each recent kernel version
Kernel Newbies
a source of various kernel-related information
Kernel coverage at LWN.net
an authoritative source of kernel-related information
Bootlin's Elixir Cross Referencer
a Linux kernel source code cross-reference * {{DEFAULTSORT:Linux Kernel Finnish inventions Free and open-source software Free software programmed in C Free system software Linus Torvalds Monolithic kernels Operating systems Software using the GPL license Unix variants
Radeon
Radeon () is a brand of computer products, including graphics processing units, random-access memory, RAM disk software, and solid-state drives, produced by Radeon Technologies Group, a division of AMD. The brand was launched in 2000 by ATI Tec ...
dynamic power management, low-latency network polling, and zswap (compressed swap cache).
The numbering change from 2.6.39 to 3.0, and from 3.19 to 4.0, involved no meaningful technical differentiation. The major version number was increased to avoid large minor numbers. Stable 3.x.y kernels were released until 3.19 in February 2015.
In April 2015, Torvalds released kernel version 4.0. By February 2015, Linux had received contributions from nearly 12,000 programmers from more than 1,200 companies, including some of the world's largest software and hardware vendors. Version 4.1 of Linux, released in June 2015, contains over 19.5 million lines of code contributed by almost 14,000 programmers.
A total of 1,991 developers, of whom 334 were first-time collaborators, added more than 553,000 lines of code to version 5.8, breaking the record previously held by version 4.9.
According to the Stack Overflow's annual Developer Survey of 2019, more than the 53% of all respondents have developed software for Linux
Linux ( or ) is a family of open-source Unix-like operating systems based on the Linux kernel, an operating system kernel first released on September 17, 1991, by Linus Torvalds. Linux is typically packaged as a Linux distribution, which i ...
and about 27% for Android
Android may refer to:
Science and technology
* Android (robot), a humanoid robot or synthetic organism designed to imitate a human
* Android (operating system), Google's mobile operating system
** Bugdroid, a Google mascot sometimes referred to ...
, although only about 25% develop with Linux-based operating systems.
Most websites run on Linux-based operating systems, and all of the world's 500 most powerful supercomputers use some kind of OS based on Linux.
Linux distributions
A Linux distribution (often abbreviated as distro) is an operating system made from a software collection that includes the Linux kernel and, often, a package management system. Linux users usually obtain their operating system by downloading on ...
bundle the kernel with system software
System software is software designed to provide a platform for other software. Examples of system software include operating systems (OS) like macOS, Linux, Android and Microsoft Windows, computational science software, game engines, search engin ...
(e.g., the GNU C Library
The GNU C Library, commonly known as glibc, is the GNU Project's implementation of the C standard library. Despite its name, it now also directly supports C++ (and, indirectly, other programming languages). It was started in the 1980s by ...
, systemd, and others Unix
Unix (; trademarked as UNIX) is a family of multitasking, multiuser computer operating systems that derive from the original AT&T Unix, whose development started in 1969 at the Bell Labs research center by Ken Thompson, Dennis Ritchie, a ...
utilities
A public utility company (usually just utility) is an organization that maintains the infrastructure for a public service (often also providing a service using that infrastructure). Public utilities are subject to forms of public control and ...
and daemons) and a wide selection of application software
Application may refer to:
Mathematics and computing
* Application software, computer software designed to help the user to perform specific tasks
** Application layer, an abstraction layer that specifies protocols and interface methods used in a ...
, but their usage share in desktops is low in comparison to other operating systems.
Android
Android may refer to:
Science and technology
* Android (robot), a humanoid robot or synthetic organism designed to imitate a human
* Android (operating system), Google's mobile operating system
** Bugdroid, a Google mascot sometimes referred to ...
, which accounts for the majority of the installed base of all operating systems for mobile devices, is responsible for the rising usage of the Linux kernel, together with its wide use in a large variety of embedded devices.
Architecture and features
Linux is a monolithic kernel
A monolithic kernel is an operating system architecture where the entire operating system is working in kernel space. The monolithic model differs from other operating system architectures (such as the microkernel architecture) in that it alone d ...
with a modular design (e.g., it can insert and remove loadable kernel modules at runtime), supporting most features once only available in closed source kernels of non-free operating systems. The rest of the article makes use of the UNIX and Unix-like operating systems convention on the official manual pages. The numbers that follow the name of commands, interfaces, and other features, have the purpose of specifying the section (i.e., the type of the OS' component or feature) they belong to (e.g., refers to a system call, while refers to a userspace library wrapper). The following list and the subsequent sections describe a non-comprehensive overview of Linux architectural design and of some of its noteworthy features.
* Concurrent computing
Concurrent computing is a form of computing in which several computations are executed '' concurrently''—during overlapping time periods—instead of ''sequentially—''with one completing before the next starts.
This is a property of a sys ...
and (with the availability of enough CPU cores for tasks that are ready to run) even true parallel execution of many processes
A process is a series or set of activities that interact to produce a result; it may occur once-only or be recurrent or periodic.
Things called a process include:
Business and management
*Business process, activities that produce a specific se ...
at once (each of them having one or more threads of execution) on SMP
SMP may refer to:
Organisations
* Scale Model Products, 1950s, acquired by Aluminum Model Toys
* School Mathematics Project, UK developer of mathematics textbooks
* '' Sekolah Menengah Pertama'', "junior high school" in Indonesia
* Shanghai Mun ...
and NUMA architectures.
* Selection and configuration of hundreds of kernel features and drivers (using one of the family of commands, before running compilation), modification of kernel parameters before booting
In computing, booting is the process of starting a computer as initiated via Computer hardware, hardware such as a button or by a software command. After it is switched on, a computer's central processing unit (CPU) has no software in its ma ...
(usually by inserting instructions into the lines of the GRUB2 menu), and fine tuning of kernel behavior at run-time (using the interface to ).
* Configuration (again using the commands) and run-time modifications of the policies (via , , and the family of syscalls) of the task schedulers that allow preemptive multitasking
In computing, preemption is the act of temporarily interrupting an executing task, with the intention of resuming it at a later time. This interrupt is done by an external scheduler with no assistance or cooperation from the task. This preemp ...
(both in user mode
A modern computer operating system usually segregates virtual memory into user space and kernel space. Primarily, this separation serves to provide memory protection and hardware protection from malicious or errant software behaviour.
Kernel ...
and, since the 2.6 series, in kernel mode
In computer science, hierarchical protection domains, often called protection rings, are mechanisms to protect data and functionality from faults (by improving fault tolerance) and malicious behavior (by providing computer security).
Compute ...
); the Completely Fair Scheduler (CFS) is the default scheduler of Linux since 2007 and it uses a red-black tree Red-black or Redblack may refer to:
* Ottawa Redblacks, a Canadian football team
* RED/BLACK concept, a concept in cryptography
* Red-black striped snake, a colubrid snake
* Red–black tree, a type of self-balancing binary search tree used in com ...
which can search, insert and delete process information (task struct A process control block (PCB) is a data structure used by computer operating systems to store all the information about a process. It is also known as a process descriptor. When a process is created (initialized or installed), the operating system c ...
) with O(log n) time complexity
In computer science, the time complexity is the computational complexity that describes the amount of computer time it takes to run an algorithm. Time complexity is commonly estimated by counting the number of elementary operations performed by ...
, where ''n'' is the number of runnable tasks.
* Advanced memory management
Memory management is a form of resource management applied to computer memory. The essential requirement of memory management is to provide ways to dynamically allocate portions of memory to programs at their request, and free it for reuse when ...
with paged virtual memory.
* Inter-process communications and synchronization mechanism.
* A virtual filesystem on top of several concrete filesystems (ext4
ext4 (fourth extended filesystem) is a journaling file system for Linux, developed as the successor to ext3.
ext4 was initially a series of backward-compatible extensions to ext3, many of them originally developed by Cluster File Systems fo ...
, Btrfs
Btrfs (pronounced as "better F S", "butter F S", "b-tree F S", or simply by spelling it out) is a computer storage format that combines a file system based on the copy-on-write (COW) principle with a logical volume manager (not to be confused ...
, XFS, JFS, FAT32
File Allocation Table (FAT) is a file system developed for personal computers. Originally developed in 1977 for use on floppy disks, it was adapted for use on hard disks and other devices. It is often supported for compatibility reasons by ...
, and many more).
* Configurable I/O schedulers, syscall that manipulates the underlying device parameters of special files (it is a non standard system call, since arguments, returns, and semantics depends on the device driver in question), support for POSIX asynchronous I/O (however, because they scale poorly with multithreaded applications, a family of Linux specific I/O system calls () had to be created for the management of asynchronous I/O contexts suitable for concurrently processing).
* OS-level virtualization
OS-level virtualization is an operating system (OS) paradigm in which the kernel allows the existence of multiple isolated user space instances, called ''containers'' (LXC, Solaris containers, Docker, Podman), ''zones'' (Solaris containers), '' ...
(with Linux-VServer), paravirtualization
In computing, paravirtualization or para-virtualization is a virtualization technique that presents a software interface to the virtual machines which is similar, yet not identical, to the underlying hardware–software interface.
The intent o ...
and hardware-assisted virtualization
In computing, hardware-assisted virtualization is a platform virtualization approach that enables efficient full virtualization using help from hardware capabilities, primarily from the host processors. A full virtualization is used to emulate a c ...
(with KVM or Xen, and using QEMU
QEMU is a free and open-source emulator (Quick EMUlator). It emulates the machine's processor through dynamic binary translation and provides a set of different hardware and device models for the machine, enabling it to run a variety of g ...
for hardware emulation); On the Xen hypervisor
A hypervisor (also known as a virtual machine monitor, VMM, or virtualizer) is a type of computer software, firmware or hardware that creates and runs virtual machines. A computer on which a hypervisor runs one or more virtual machines is called ...
, the Linux kernel provides support to build Linux distributions (such as openSuSE Leap and many others) that work as ''Dom0'', that are virtual machine host servers that provide the management environment for the user's virtual machines (''DomU'').
*I/O Virtualization with VFIO and SR-IOV. Virtual Function I/O (VFIO) exposes direct device access to user space in a secure memory (IOMMU) protected environment. With VFIO, a VM Guest can directly access hardware devices on the VM Host Server. This technique improves performance, if compared both to Full virtualization and Paravirtualization. However, with VFIO, devices cannot be shared with multiple VM guests. Single Root I/O Virtualization (SR-IOV) combines the performance gains of VFIO and the ability to share a device with several VM Guests (but it requires special hardware that must be capable to appear to two or more VM guests as different devices).
* Security mechanisms for discretionary and mandatory access control (SELinux, AppArmor, POSIX ACLs, and others).
* Several types of layered communication protocol
A communication protocol is a system of rules that allows two or more entities of a communications system to transmit information via any kind of variation of a physical quantity. The protocol defines the rules, syntax, semantics and synchroniza ...
s (including the Internet protocol suite
The Internet protocol suite, commonly known as TCP/IP, is a framework for organizing the set of communication protocols used in the Internet and similar computer networks according to functional criteria. The foundational protocols in the sui ...
).
* Asymmetric multiprocessing via the RPMsg subsystem.
Most Device driver
In computing, a device driver is a computer program that operates or controls a particular type of device that is attached to a computer or automaton. A driver provides a software interface to hardware devices, enabling operating systems and o ...
s and kernel extensions run in kernel space
A modern computer operating system usually segregates virtual memory into user space and kernel space. Primarily, this separation serves to provide memory protection and hardware protection from malicious or errant software behaviour.
Kerne ...
( ring 0 in many CPU
A central processing unit (CPU), also called a central processor, main processor or just processor, is the electronic circuitry that executes instructions comprising a computer program. The CPU performs basic arithmetic, logic, controlling, and ...
architectures), with full access to the hardware. Some exceptions run in user space
A modern computer operating system usually segregates virtual memory into user space and kernel space. Primarily, this separation serves to provide memory protection and hardware protection from malicious or errant software behaviour.
Kerne ...
; notable examples are filesystems based on FUSE
Fuse or FUSE may refer to:
Devices
* Fuse (electrical), a device used in electrical systems to protect against excessive current
** Fuse (automotive), a class of fuses for vehicles
* Fuse (hydraulic), a device used in hydraulic systems to prote ...
/CUSE, and parts of UIO. Furthermore, the X Window System
The X Window System (X11, or simply X) is a windowing system for bitmap displays, common on Unix-like operating systems.
X provides the basic framework for a GUI environment: drawing and moving windows on the display device and interacting ...
and Wayland, the windowing system and display server protocols that most people use with Linux, do not run within the kernel. Differently, the actual interfacing with GPUs of graphics card
A graphics card (also called a video card, display card, graphics adapter, VGA card/VGA, video adapter, display adapter, or mistakenly GPU) is an expansion card which generates a feed of output images to a display device, such as a computer mo ...
s is an in-kernel subsystem called Direct Rendering Manager
The Direct Rendering Manager (DRM) is a subsystem of the Linux kernel responsible for interfacing with GPUs of modern video cards. DRM exposes an API that user-space programs can use to send commands and data to the GPU and perform operations ...
(DRM).
Unlike standard monolithic kernels, device drivers are easily configured as modules, and loaded or unloaded while the system is running and can also be pre-empted under certain conditions in order to handle hardware interrupts correctly and to better support symmetric multiprocessing
Symmetric multiprocessing or shared-memory multiprocessing (SMP) involves a multiprocessor computer hardware and software architecture where two or more identical processors are connected to a single, shared main memory, have full access to all ...
. By choice, Linux has no stable device driver application binary interface
In computer software, an application binary interface (ABI) is an interface between two binary program modules. Often, one of these modules is a library or operating system facility, and the other is a program that is being run by a user.
An ...
.
Linux typically makes use of memory protection
Memory protection is a way to control memory access rights on a computer, and is a part of most modern instruction set architectures and operating systems. The main purpose of memory protection is to prevent a process from accessing memory that h ...
and virtual memory
In computing, virtual memory, or virtual storage is a memory management technique that provides an "idealized abstraction of the storage resources that are actually available on a given machine" which "creates the illusion to users of a very ...
and can also handle non-uniform memory access
Non-uniform memory access (NUMA) is a computer memory design used in multiprocessing, where the memory access time depends on the memory location relative to the processor. Under NUMA, a processor can access its own local memory faster than non ...
, however the project has absorbed μClinux
μClinux is a variation of the Linux kernel, previously maintained as a fork, that targets microcontrollers without a memory management unit (MMU). It was integrated into the mainline kernel as of 2.5.46; the project continues to develop patc ...
which also makes it possible to run Linux on microcontroller
A microcontroller (MCU for ''microcontroller unit'', often also MC, UC, or μC) is a small computer on a single VLSI integrated circuit (IC) chip. A microcontroller contains one or more CPUs ( processor cores) along with memory and programma ...
s without virtual memory.
The hardware is represented in the file hierarchy. User applications interact with device drivers via entries in the or directories. Processes information as well are mapped to the file system through the directory.
Interfaces
Linux is a clone of UNIX, and aims toward POSIX
The Portable Operating System Interface (POSIX) is a family of standards specified by the IEEE Computer Society for maintaining compatibility between operating systems. POSIX defines both the system- and user-level application programming inte ...
and Single UNIX Specification compliance. The kernel also provides system calls and other interfaces that are Linux-specific. In order to be included in the official kernel, the code must comply with a set of licensing rules.
The Linux Application binary interface
In computer software, an application binary interface (ABI) is an interface between two binary program modules. Often, one of these modules is a library or operating system facility, and the other is a program that is being run by a user.
An ...
(ABI) between the kernel and the user space has four degrees of stability (stable, testing, obsolete, removed); however, the system call
In computing, a system call (commonly abbreviated to syscall) is the programmatic way in which a computer program requests a service from the operating system on which it is executed. This may include hardware-related services (for example, acc ...
s are expected to never change in order to not break the userspace
A modern computer operating system usually segregates virtual memory into user space and kernel space. Primarily, this separation serves to provide memory protection and hardware protection from malicious or errant software behaviour.
Kernel ...
programs that rely on them.
Loadable kernel module
In computing, a loadable kernel module (LKM) is an object file that contains code to extend the running kernel, or so-called ''base kernel'', of an operating system. LKMs are typically used to add support for new hardware (as device drivers) and ...
s (LKMs), by design, cannot rely on a stable ABI. Therefore, they must always be recompiled whenever a new kernel executable is installed in a system, otherwise they will not be loaded. In-tree drivers that are configured to become an integral part of the kernel executable ( vmlinux) are statically linked by the building process.
There is also no guarantee of stability of source-level in-kernel API and, because of this, device driver
In computing, a device driver is a computer program that operates or controls a particular type of device that is attached to a computer or automaton. A driver provides a software interface to hardware devices, enabling operating systems and o ...
s code, as well as the code of any other kernel subsystem, must be kept updated with kernel evolution. Any developer who makes an API change is required to fix any code that breaks as the result of their change.
Kernel-to-userspace API
The set of theLinux kernel API
The Linux kernel provides several interfaces to user-space applications that are used for different purposes and that have different properties by design. There are two types of application programming interface (API) in the Linux kernel tha ...
that regards the interfaces exposed to user applications is fundamentally composed of UNIX and Linux-specific system call
In computing, a system call (commonly abbreviated to syscall) is the programmatic way in which a computer program requests a service from the operating system on which it is executed. This may include hardware-related services (for example, acc ...
s. A system call is an entry point into the Linux kernel. For example, among the Linux-specific ones there is the family of the system calls. Most extensions must be enabled by defining the _GNU_SOURCE macro
Macro (or MACRO) may refer to:
Science and technology
* Macroscopic, subjects visible to the eye
* Macro photography, a type of close-up photography
* Image macro, a picture with text superimposed
* Monopole, Astrophysics and Cosmic Ray Observat ...
in a header file
Many programming languages and other computer files have a directive, often called include (sometimes copy or import), that causes the contents of the specified file to be inserted into the original file. These included files are called copybooks ...
or when the user-land code is being compiled.
System calls can only be invoked by using assembly instructions which enable the transition from unprivileged user space to privileged kernel space in ring 0. For this reason, the C standard library (libC) acts as a wrapper to most Linux system calls, by exposing C functions that, only whether it is needed, can transparently enter into the kernel which will execute on behalf of the calling process. For those system calls not exposed by libC, e.g. the ''fast userspace mutex'' (futex
In computing, a futex (short for "fast userspace mutex") is a kernel system call that programmers can use to implement basic locking, or as a building block for higher-level locking abstractions such as semaphores and POSIX mutexes or condit ...
), the library provides a function called which can be used to explicitly invoke them.
Pseudo filesystems (e.g., the sysfs
sysfs is a pseudo file system provided by the Linux kernel that exports information about various kernel subsystems, hardware devices, and associated device drivers from the kernel's device model to user space through virtual files. In addit ...
and procfs
The proc filesystem (procfs) is a special filesystem in Unix-like operating systems that presents information about processes and other system information in a hierarchical file-like structure, providing a more convenient and standardized meth ...
filesystems) and special file
In Unix-like operating systems, a device file or special file is an interface to a device driver that appears in a file system as if it were an ordinary file. There are also special files in DOS, OS/2, and Windows. These special files al ...
s (e.g., /dev/random, /dev/sda, /dev/tty, and many others) constitute another layer of interface to kernel data structures representing hardware or logical (software) devices.
Kernel-to-userspace ABI
Because of the differences existing between the hundreds of various implementations of the Linux OS, executable objects, even though they are compiled, assembled, and linked for running on a specific hardware architecture (that is, they use theISA
Isa or ISA may refer to:
Places
* Isa, Amur Oblast, Russia
* Isa, Kagoshima, Japan
* Isa, Nigeria
* Isa District, Kagoshima, former district in Japan
* Isa Town, middle class town located in Bahrain
* Mount Isa, Queensland, Australia
* Mount Is ...
of the target hardware), often cannot run on different Linux Distributions. This issue is mainly due to distribution-specific configurations and a set of patches applied to the code of the Linux kernel, differences in system libraries, services (daemons), filesystem hierarchies, and environment variables.
The main standard concerning application and binary compatibility of Linux distributions is the Linux Standard Base (LSB). However, the LSB goes beyond what concerns the Linux kernel, because it also defines the desktop specifications, the X libraries and Qt that have little to do with it. The LSB version 5 is built upon several standards and drafts (POSIX, SUS, X/Open, File System Hierarchy (FHS), and others).
The parts of the LSB largely relevant to the kernel are the ''General ABI'' (gABI), especially the System V ABI and the Executable and Linking Format
In computing, the Executable and Linkable FormatTool Interface Standard (TIS) Portable Formats SpecificationVersion 1.1'' (October 1993) (ELF, formerly named Extensible Linking Format), is a common standard file format for executable files, obj ...
(ELF), and the ''Processor Specific ABI'' (psABI), for example the ''Core Specification for X86-64.''
The standard ABI for how x86_64 user programs invoke system calls is to load the syscall number into the ''rax'' register, and the other parameters into ''rdi'', ''rsi'', ''rdx'', ''r10'', ''r8'', and ''r9'', and finally to put the ''syscall'' assembly instruction in the code.
In-kernel API
There are several kernel internal APIs utilized between the different subsystems. Some are available only within the kernel subsystems, while a somewhat limited set of in-kernel symbols (i.e., variables, data structures, and functions) is exposed also to dynamically loadable modules (e.g., device drivers loaded on demand) whether they're exported with the and macros (the latter reserved to modules released under a GPL-compatible license).
Linux provides in-kernel APIs that manipulate data structures (e.g., linked list
In computer science, a linked list is a linear collection of data elements whose order is not given by their physical placement in memory. Instead, each element points to the next. It is a data structure consisting of a collection of nodes whi ...
s, radix tree
In computer science, a radix tree (also radix trie or compact prefix tree or compressed trie) is a data structure that represents a space-optimized trie (prefix tree) in which each node that is the only child is merged with its parent. The resul ...
s, red-black trees Red-black or Redblack may refer to:
* Ottawa Redblacks, a Canadian football team
* RED/BLACK concept, a concept in cryptography
* Red-black striped snake, a colubrid snake
* Red–black tree, a type of self-balancing binary search tree used in com ...
, queues) or perform common routines (e.g., copy data from and to user space, allocate memory, print lines to the system log, and so on) that have remained stable at least since Linux version 2.6.
In-kernel APIs include libraries of low-level common services used by device drivers:
* SCSI
Small Computer System Interface (SCSI, ) is a set of standards for physically connecting and transferring data between computers and peripheral devices. The SCSI standards define commands, protocols, electrical, optical and logical interface ...
Interfaces and libATA respectively, a peer-to-peer packet based communication protocol for storage devices attached to USB, SATA, SAS, Fibre Channel, FireWire, ATAPI device, and an in-kernel library to support TA host controllers and devices.
* Direct Rendering Manager
The Direct Rendering Manager (DRM) is a subsystem of the Linux kernel responsible for interfacing with GPUs of modern video cards. DRM exposes an API that user-space programs can use to send commands and data to the GPU and perform operations ...
(DRM) and Kernel Mode Setting
The Direct Rendering Manager (DRM) is a subsystem of the Linux kernel responsible for interfacing with GPUs of modern video cards. DRM exposes an API that user-space programs can use to send commands and data to the GPU and perform operations su ...
(KMS) for interfacing with GPUs and supporting the needs of modern 3D-accelerated video hardware, and for setting screen resolution, color depth and refresh rate
* DMA
DMA may refer to:
Arts
* ''DMA'' (magazine), a defunct dance music magazine
* Dallas Museum of Art, an art museum in Texas, US
* Danish Music Awards, an award show held in Denmark
* BT Digital Music Awards, an annual event in the UK
* Doctor of M ...
buffers (DMA-BUF
The Direct Rendering Manager (DRM) is a subsystem of the Linux kernel responsible for interfacing with GPUs of modern video cards. DRM exposes an API that user-space programs can use to send commands and data to the GPU and perform operations su ...
) for sharing buffers for hardware direct memory access across multiple device drivers and subsystems
* Video4Linux for video capture hardware
* Advanced Linux Sound Architecture (ALSA) for sound cards
* New API for network interface controller
A network interface controller (NIC, also known as a network interface card, network adapter, LAN adapter or physical network interface, and by similar terms) is a computer hardware component that connects a computer to a computer network.
E ...
s
* mac80211 In-kernel ABI
The Linux developers chose not to maintain a stable in-kernel ABI. Modules compiled for a specific version of the kernel cannot be loaded into another version without being recompiled, assuming that the in-kernel API has remained the same at the source level; otherwise, the module code must also be modified accordingly.Processes and threads
Linux creates processes by means of the or by the newer system calls. Depending on the given parameters, the new entity can share most or none of the resources of the caller. These syscalls can create new entities ranging from new independent processes (each having a special identifier called ''TGID'' within the ''task_struct'' data structure in kernel space, although that same identifier is called ''PID'' in userspace), to new threads of execution within the calling process (by using the parameter). In this latter case the new entity owns the same ''TGID'' of the calling process and consequently has also the same ''PID'' in userspace. If the executable is dynamically linked to shared libraries, adynamic linker
In computing, a dynamic linker is the part of an operating system that loads and links the shared libraries needed by an executable when it is executed (at " run time"), by copying the content of libraries from persistent storage to RAM, filli ...
(for ELF objects, it is typically ) is used to find and load the needed objects, prepare the program to run and then run it.
The Native POSIX Thread Library, simply known as the NPTL, provides the standard POSIX threads interface (''pthreads'') to userspace. Whenever a new thread is created using the pthread_create(3) POSIX interface, the family of system calls must also be given the address of the function that the new thread must jump to. The Linux kernel provides the (acronym for "Fast user-space mutexes") mechanisms for fast user-space locking and synchronization; the majority of the operations are performed in userspace but it may be necessary to communicate with the kernel using the system call.
A very special category of threads is the so-called ''kernel threads''. They must not be confused with the above-mentioned threads of execution of the user's processes. Kernel threads exist only in kernel space and their only purpose is to concurrently run kernel tasks.
Differently, whenever an independent process is created, the syscalls return exactly to the next instruction of the same program, concurrently in ''parent'' process and in ''child's'' one (i.e., one program, two processes). Different return values (one per process) enable the program to know in which of the two processes it is currently executing. Programs need this information because the child process, a few steps after process duplication, usually invokes the system call (possibly via the family of wrapper functions in glibC) and replace the program that is currently being run by the calling process with a new program, with newly initialized stack, heap, and (initialized and uninitialized) data segments. When it is done, it results in two processes that run two different programs.
Depending on the effective user id (''euid''), and on the effective group id (''egid''), a process running with user zero privileges (''root'', the system administrator, owns the identifier 0) can perform everything (e.g., kill all the other processes or recursively wipe out whole filesystems), instead non zero user processes cannot. divides the privileges traditionally associated with superuser into distinct units, which can be independently enabled and disabled by the parent process or dropped by the child itself.
Scheduling and preemption
The Linux scheduler is modular, in the sense that it enables different scheduling classes and policies. Scheduler classes are plugable scheduler algorithms that can be registered with the base scheduler code. Each class schedules different types of processes. The core code of the scheduler iterates over each class in order of priority and chooses the highest priority scheduler that has a schedulable entity of type struct sched_entity ready to run. Entities may be threads, group of threads, and even all the processes of a specific user. Linux provides both ''user preemption'' as well as full ''kernel preemption''. Preemption reduces latency, increases responsiveness, and makes Linux more suitable for desktop and real-time applications. For normal tasks, by default, the kernel uses the Completely Fair Scheduler (CFS) class, introduced in the 2.6.23 version of the kernel. Internally this default-scheduler class is defined in a macro of a C header asSCHED_NORMAL. In other POSIX kernels, a similar policy known as SCHED_OTHER allocates CPU timeslices (i.e, it assigns absolute slices of the processor time depending on either predetermined or dynamically computed priority of each process). The Linux CFS does away with absolute timeslices and assigns a fair proportion of CPU time, as a function of parameters like the total number of runnable processes and the time they have already run; this function also takes into account a kind of weight that depends on their relative priorities (nice values).
With user preemption, the kernel scheduler can replace the current process with the execution of a context switch
In computing, a context switch is the process of storing the state of a process or thread, so that it can be restored and resume execution at a later point, and then restoring a different, previously saved, state. This allows multiple processes ...
to a different one that therefore acquires the computing resources for running (CPU, memory, and more). It makes it according to the CFS CFS is an acronym for:
Organizations
* Canadian Federation of Students
* Canadian Forest Service
* Center for Financial Studies, a research institute affiliated with Goethe University Frankfurt
* Center for Subjectivity Research, a research insti ...
algorithm (in particular, it uses a variable called for sorting entities and then chooses the one that has the smaller vruntime, - i.e., the schedulable entity that has had the least share of CPU time), to the active scheduler policy and to the relative priorities. With kernel preemption, the kernel can preempt itself when an interrupt handler returns, when kernel tasks block, and whenever a subsystem explicitly calls the schedule() function.
The kernel also contains two POSIX-compliant real-time scheduling classes named SCHED_FIFO (realtime first-in-first-out) and SCHED_RR (realtime round-robin), both of which take precedence over the default class. An additional scheduling policy known as SCHED DEADLINE, implementing the earliest deadline first algorithm (EDF), was added in kernel version 3.14, released on 30 March 2014. SCHED_DEADLINE takes precedence over all the other scheduling classes.
Real-time PREEMPT_RT patches, included into the mainline Linux since version 2.6, provide a deterministic scheduler, the removal of preemption and interrupts disabling (where possible), PI Mutexes (i.e., locking primitives that avoid priority inversion), support for high precision event timers ( HPET), preemptive Read-copy-update, (forced) IRQ threads, and other minor features.
Concurrency and synchronization
The kernel has different causes of concurrency (e.g., interrupts, bottom halves, preemption of kernel and users tasks, symmetrical multiprocessing). For protecting critical regions (sections of code that must be executed atomically), shared memory locations (likeglobal variable
In computer programming, a global variable is a variable with global scope, meaning that it is visible (hence accessible) throughout the program, unless shadowed. The set of all global variables is known as the ''global environment'' or ''global ...
s and other data structures with global scope), and regions of memory that are asynchronously modifiable by hardware (e.g., having the C volatile type qualifier), Linux provides a large set of tools. They consist of atomic types (which can only be manipulated by a set of specific operators), spinlocks, semaphores, mutexes, and lockless algorithms (e.g., RCUs). Most lock-less algorithms are built on top of memory barrier
In computing, a memory barrier, also known as a membar, memory fence or fence instruction, is a type of barrier instruction that causes a central processing unit (CPU) or compiler to enforce an ordering constraint on memory operations issued ...
s for the purpose of enforcing memory ordering and prevent undesired side effects due to compiler optimization
In computing, an optimizing compiler is a compiler that tries to minimize or maximize some attributes of an executable computer program. Common requirements are to minimize a program's execution time, memory footprint, storage size, and power c ...
.
PREEMPT_RT code included in mainline Linux provide ''RT-mutexes'', a special kind of Mutex which do not disable preemption and have support for priority inheritance. Almost all locks are changed into sleeping locks when using configuration for realtime operation. Priority inheritance avoids priority inversion by granting a low-priority task which holds a contended lock the priority of a higher-priority waiter until that lock is released.
Linux includes a kernel lock validator called ''Lockdep''.
Interrupts management
The management of theinterrupt
In digital computers, an interrupt (sometimes referred to as a trap) is a request for the processor to ''interrupt'' currently executing code (when permitted), so that the event can be processed in a timely manner. If the request is accepted ...
s, although it could be seen as a single job, is divided in two separate parts. This split in two is due to the different time constraints and to the synchronization needs of the tasks whose the management is composed of. The first part is made up of an asynchronous interrupt service routine that in Linux is known as the ''top half'', while the second part is carried out by one of three types of the so-called ''bottom halves'' (''softirq'', ''tasklets,'' and ''work queues''). Linux interrupts service routines can be nested (i.e., a new IRQ can trap into a high priority ISR that preempts any other lower priority ISRs).
Memory management
Memory management in Linux is a complex topic. First of all, the kernel is not pageable (i.e., it is always resident in physical memory and cannot be swapped to the disk). In the kernel there is no memory protection (no ''SIGSEGV'' signals, unlike in userspace), therefore memory violations lead to instability and system crashes. Linux implementsvirtual memory
In computing, virtual memory, or virtual storage is a memory management technique that provides an "idealized abstraction of the storage resources that are actually available on a given machine" which "creates the illusion to users of a very ...
with 4 and 5-levels page tables. As said, only user memory space is always pageable. It maintains information about each page frame
A page, memory page, or virtual page is a fixed-length contiguous block of virtual memory, described by a single entry in the page table. It is the smallest unit of data for memory management in a virtual memory operating system. Similarly, a ...
of RAM in apposite data structures (of type ) that are populated immediately after boots and that are kept until shutdown, regardless of them being or not associated with virtual pages. Furthermore, it classifies all page frames in zones, according to their architecture dependent constraints and intended use. For example, pages reserved for DMA operations are in ZONE_DMA, pages that are not permanently mapped to virtual addresses are in ZONE_HIGHMEM (in x86_32 architecture this zone is for physical addresses above 896 MB, while x86_64 does not need it because x86_64 can permanently map physical pages that reside in higher addresses), and all that remains (with the exception of other less used classifications) is in ZONE_NORMAL.
Small chunks of memory can be dynamically allocated via the family of kmalloc() API and freed with the appropriate variant of kfree(). vmalloc() and kvfree() are used for large virtually contiguous chunks. alloc_pages() allocates the desired number of entire pages.
Kernel includes SLAB, SLUB and SLOB allocators as configurable alternatives. SLUB is the newest and it is also the default allocator. It aims for simplicity and efficiency. SLUB has been made PREEMPT_RT compatible.
Supported architectures
While not originally designed to be portable, Linux is now one of the most widely ported operating system kernels, running on a diverse range of systems from the ARM architecture
ARM (stylised in lowercase as arm, formerly an acronym for Advanced RISC Machines and originally Acorn RISC Machine) is a family of reduced instruction set computer (RISC) instruction set architectures for computer processors, configure ...
to IBM z/Architecture mainframe computer
A mainframe computer, informally called a mainframe or big iron, is a computer used primarily by large organizations for critical applications like bulk data processing for tasks such as censuses, industry and consumer statistics, enterpris ...
s. The first port was performed on the Motorola 68000
The Motorola 68000 (sometimes shortened to Motorola 68k or m68k and usually pronounced "sixty-eight-thousand") is a 16/32-bit complex instruction set computer (CISC) microprocessor, introduced in 1979 by Motorola Semiconductor Products Sect ...
platform. The modifications to the kernel were so fundamental that Torvalds viewed the Motorola version as a fork
In cutlery or kitchenware, a fork (from la, furca ' pitchfork') is a utensil, now usually made of metal, whose long handle terminates in a head that branches into several narrow and often slightly curved tines with which one can spear foods ...
and a "Linux-like operating system". However, that moved Torvalds to lead a major restructure of the code to facilitate porting to more computing architectures. The first Linux that, in a single source tree, had code for more than i386 alone, supported the DEC Alpha AXP 64-bit platform.
Linux runs as the main operating system on IBM's Summit
A summit is a point on a surface that is higher in elevation than all points immediately adjacent to it. The topographic terms acme, apex, peak (mountain peak), and zenith are synonymous.
The term (mountain top) is generally used only for a m ...
; , all of the world's 500 fastest supercomputers run some operating system based on the Linux kernel, a big change from 1998 when the first Linux supercomputer got added to the list.
Linux has also been ported to various handheld devices such as Apple's iPhone 3G and iPod
The iPod is a discontinued series of portable media players and multi-purpose mobile devices designed and marketed by Apple Inc. The first version was released on October 23, 2001, about months after the Macintosh version of iTunes ...
.
Supported devices
In 2007, the LKDDb project has been started to build a comprehensive database of hardware and protocols known by Linux kernels. The database is built automatically by static analysis of the kernel sources. Later in 2014, the Linux Hardware project was launched to automatically collect a database of all tested hardware configurations with the help of users of various Linux distributions.Live patching
Rebootless updates can even be applied to the kernel by usinglive patching
A patch is a set of changes to a computer program or its supporting data designed to update, fix, or improve it. This includes fixing security vulnerabilities and other bugs, with such patches usually being called bugfixes or bug fixes. Patches ...
technologies such as Ksplice, kpatch and kGraft. Minimalistic foundations for live kernel patching were merged into the Linux kernel mainline in kernel version 4.0, which was released on 12 April 2015. Those foundations, known as ''livepatch'' and based primarily on the kernel's ftrace functionality, form a common core capable of supporting hot patching by both kGraft and kpatch, by providing an application programming interface (API) for kernel modules that contain hot patches and an application binary interface
In computer software, an application binary interface (ABI) is an interface between two binary program modules. Often, one of these modules is a library or operating system facility, and the other is a program that is being run by a user.
An ...
(ABI) for the userspace management utilities. However, the common core included into Linux kernel 4.0 supports only the x86 architecture and does not provide any mechanisms for ensuring function-level consistency while the hot patches are applied. , there is ongoing work on porting kpatch and kGraft to the common live patching core provided by the Linux kernel mainline.
Security
Kernel bugs present potential security issues. For example, they may allow for privilege escalation or createdenial-of-service attack
In computing, a denial-of-service attack (DoS attack) is a cyber-attack in which the perpetrator seeks to make a machine or network resource unavailable to its intended users by temporarily or indefinitely disrupting services of a host conn ...
vectors. Over the years, numerous bugs affecting system security were found and fixed. New features are frequently implemented to improve the kernel's security.
Capabilities(7) have already been introduced in the section about the processes and threads. Android makes use of them and systemd gives administrators detailed control over the capabilities of processes.
Linux offers a wealth of mechanisms to reduce kernel attack surface and improve security which are collectively known as the Linux Security Modules (LSM). They comprise the Security-Enhanced Linux
Security-Enhanced Linux (SELinux) is a Linux kernel security module that provides a mechanism for supporting access control security policies, including mandatory access controls (MAC).
SELinux is a set of kernel modifications and user-space ...
(SELinux) module, whose code has been originally developed and then released to the public by the NSA, and AppArmor among others. SELinux is now actively developed and maintained on GitHub
GitHub, Inc. () is an Internet hosting service for software development and version control using Git. It provides the distributed version control of Git plus access control, bug tracking, software feature requests, task management, co ...
. SELinux and AppArmor provide support to access control security policies, including mandatory access control (MAC), though they profoundly differ in complexity and scope.
Another security feature is the Seccomp BPF (SECure COMPuting with Berkeley Packet Filters) which works by filtering parameters and reducing the set of system calls available to user-land applications.
Critics have accused kernel developers of covering up security flaws, or at least not announcing them; in 2008, Linus Torvalds responded to this with the following:
Linux distributions typically release security updates to fix vulnerabilities in the Linux kernel. Many offer long-term support
Long-term support (LTS) is a product lifecycle management policy in which a stable release of computer software is maintained for a longer period of time than the standard edition. The term is typically reserved for open-source software, where it ...
releases that receive security updates for a certain Linux kernel version for an extended period of time.
Development
Developer community
The community of Linux kernel developers comprises about 5000–6000 members. According to the "2017 State of Linux Kernel Development", a study issued by the Linux Foundation, covering the commits for the releases 4.8 to 4.13, about 1500 developers were contributing from about 200-250 companies on average. The top 30 developers contributed a little more than 16% of the code. For companies, the top contributors are Intel (13.1%) and Red Hat (7.2%), Linaro (5.6%), IBM (4.1%), the second and fifth places are held by the 'none' (8.2%) and 'unknown' (4.1%) categories. As with many large open-source software projects, developers are required to adhere to the Contributor Covenant, acode of conduct
A code of conduct is a set of rules outlining the norms, rules, and responsibilities or proper practices of an individual party or an organization.
Companies' codes of conduct
A company code of conduct is a set of rules which is commonly writt ...
intended to address harassment of minority contributors. Additionally, to prevent offense the use of inclusive terminology within the source code is mandated.
Source code management
The Linux development community uses Git to manage thesource code
In computing, source code, or simply code, is any collection of code, with or without comment (computer programming), comments, written using a human-readable programming language, usually as plain text. The source code of a Computer program, p ...
. Git users clone the latest version of Torvalds' tree with and keep it up to date using . Contributions are submitted as patches, in the form of text messages on the LKML (and often also on other mailing lists dedicated to particular subsystems). The patches must conform to a set of rules and to a formal language that, among other things, describes which lines of code are to be deleted and what others are to be added to the specified files. These patches can be automatically processed so that system administrators can apply them in order to make just some changes to the code or to incrementally upgrade to the next version. Linux is distributed also in GNU zip (gzip) and bzip2 formats.
Submitting code to the kernel
A developer who wants to change the Linux kernel starts with developing and testing that change. Depending on how significant the change is and how many subsystems it modifies, the change will either be submitted as a single patch or in multiple patches ofsource code
In computing, source code, or simply code, is any collection of code, with or without comment (computer programming), comments, written using a human-readable programming language, usually as plain text. The source code of a Computer program, p ...
. In case of a single subsystem that is maintained by a single maintainer, these patches are sent as e-mails to the maintainer of the subsystem with the appropriate mailing list in Cc. The maintainer and the readers of the mailing list will review the patches and provide feedback. Once the review process has finished the subsystem maintainer accepts the patches in the relevant Git kernel tree. If the changes to the Linux kernel are bug fixes that are considered important enough, a pull request for the patches will be sent to Torvalds within a few days. Otherwise, a pull request will be sent to Torvalds during the next merge window. The merge window usually lasts two weeks and starts immediately after the release of the previous kernel version. The Git kernel source tree names all developers who have contributed to the Linux kernel in the ''Credits'' directory and all subsystem maintainers are listed in ''Maintainers''.
Programming language and coding style
Linux is written in a specialC programming language
''The C Programming Language'' (sometimes termed ''K&R'', after its authors' initials) is a computer programming book written by Brian Kernighan and Dennis Ritchie, the latter of whom originally designed and implemented the language, as well as ...
supported by GCC, a compiler that extends in many ways the C standard, for example using inline sections of code written in the assembly language (in GCC's "AT&T-style" syntax) of the target architecture. Since 2002 all the code must adhere to the 21 rules comprising the ''Linux Kernel Coding Style.''
GNU toolchain
TheGNU Compiler Collection
The GNU Compiler Collection (GCC) is an optimizing compiler produced by the GNU Project supporting various programming languages, hardware architectures and operating systems. The Free Software Foundation (FSF) distributes GCC as free sof ...
(GCC or GNU cc) is the default compiler
In computing, a compiler is a computer program that translates computer code written in one programming language (the ''source'' language) into another language (the ''target'' language). The name "compiler" is primarily used for programs that ...
for the mainline Linux sources and it is invoked by a utility called make. Then, the GNU Assembler
The GNU Assembler, commonly known as gas or as, is the assembler developed by the GNU Project. It is the default back-end of GCC. It is used to assemble the GNU operating system and the Linux kernel, and various other software. It is a part o ...
(more often called GAS or GNU as) outputs the object file
An object file is a computer file containing object code, that is, machine code output of an assembler or compiler. The object code is usually relocatable, and not usually directly executable. There are various formats for object files, and the ...
s from the GCC generated assembly
Assembly may refer to:
Organisations and meetings
* Deliberative assembly, a gathering of members who use parliamentary procedure for making decisions
* General assembly, an official meeting of the members of an organization or of their representa ...
code. Finally, the GNU Linker (GNU ld) is used to produce a statically linked executable kernel file called . Both and are part of GNU Binary Utilities (binutils). The above-mentioned tools are collectively known as the GNU toolchain
The GNU toolchain is a broad collection of programming tools produced by the GNU Project. These tools form a toolchain (a suite of tools used in a serial manner) used for developing software applications and operating systems.
The GNU toolchain ...
.
Compiler compatibility
GCC was for a long time the only compiler capable of correctly building Linux. In 2004,Intel
Intel Corporation is an American multinational corporation and technology company headquartered in Santa Clara, California, Santa Clara, California. It is the world's largest semiconductor chip manufacturer by revenue, and is one of the devel ...
claimed to have modified the kernel so that its C compiler was also capable of compiling it. There was another such reported success in 2009, with a modified 2.6.22 version.
Since 2010, effort has been underway to build Linux with Clang
Clang is a compiler front end for the C, C++, Objective-C, and Objective-C++ programming languages, as well as the OpenMP, OpenCL, RenderScript, CUDA, and HIP frameworks. It acts as a drop-in replacement for the GNU Compiler Collection ...
, an alternative compiler for the C language; as of 12 April 2014, the official kernel could almost be compiled by Clang. The project dedicated to this effort is named ''LLVMLinux'' after the LLVM
LLVM is a set of compiler and toolchain technologies that can be used to develop a front end for any programming language and a back end for any instruction set architecture. LLVM is designed around a language-independent intermediate repre ...
compiler infrastructure upon which Clang is built. LLVMLinux does not aim to fork either Linux or the LLVM, therefore it is a meta-project composed of patches that are eventually submitted to the upstream projects. By enabling Linux to be compiled by Clang, developers may benefit from shorter compilation times.
In 2017, developers completed upstreaming patches to support building the Linux kernel with Clang
Clang is a compiler front end for the C, C++, Objective-C, and Objective-C++ programming languages, as well as the OpenMP, OpenCL, RenderScript, CUDA, and HIP frameworks. It acts as a drop-in replacement for the GNU Compiler Collection ...
in the 4.15 release, having backported support for X86-64
x86-64 (also known as x64, x86_64, AMD64, and Intel 64) is a 64-bit version of the x86 instruction set, first released in 1999. It introduced two new modes of operation, 64-bit mode and compatibility mode, along with a new 4-level paging ...
and AArch64
AArch64 or ARM64 is the 64-bit extension of the ARM architecture family.
It was first introduced with the Armv8-A architecture. Arm releases a new extension every year.
ARMv8.x and ARMv9.x extensions and features
Announced in October 2011, ...
to the 4.4, 4.9, and 4.14 branches of the stable kernel tree. Google's Pixel 2 shipped with the first Clang
Clang is a compiler front end for the C, C++, Objective-C, and Objective-C++ programming languages, as well as the OpenMP, OpenCL, RenderScript, CUDA, and HIP frameworks. It acts as a drop-in replacement for the GNU Compiler Collection ...
built Linux
Linux ( or ) is a family of open-source Unix-like operating systems based on the Linux kernel, an operating system kernel first released on September 17, 1991, by Linus Torvalds. Linux is typically packaged as a Linux distribution, which i ...
kernel, though patches for Pixel (1st generation) did exist. 2018 saw ChromeOS
ChromeOS, sometimes stylized as chromeOS and formerly styled as Chrome OS, is a Linux-based operating system designed by Google. It is derived from the open-source ChromiumOS and uses the Google Chrome web browser as its principal user interfac ...
move to building kernels with Clang
Clang is a compiler front end for the C, C++, Objective-C, and Objective-C++ programming languages, as well as the OpenMP, OpenCL, RenderScript, CUDA, and HIP frameworks. It acts as a drop-in replacement for the GNU Compiler Collection ...
by default, while Android (operating system)
Android is a mobile operating system based on a modified version of the Linux kernel and other open-source software, designed primarily for touchscreen mobile devices such as smartphones and tablets. Android is developed by a consortium of d ...
made Clang
Clang is a compiler front end for the C, C++, Objective-C, and Objective-C++ programming languages, as well as the OpenMP, OpenCL, RenderScript, CUDA, and HIP frameworks. It acts as a drop-in replacement for the GNU Compiler Collection ...
and LLVM
LLVM is a set of compiler and toolchain technologies that can be used to develop a front end for any programming language and a back end for any instruction set architecture. LLVM is designed around a language-independent intermediate repre ...
's linker LLD required for kernel builds in 2019. Google
Google LLC () is an American Multinational corporation, multinational technology company focusing on Search Engine, search engine technology, online advertising, cloud computing, software, computer software, quantum computing, e-commerce, ar ...
moved its production kernel used throughout its datacenters to being built with Clang
Clang is a compiler front end for the C, C++, Objective-C, and Objective-C++ programming languages, as well as the OpenMP, OpenCL, RenderScript, CUDA, and HIP frameworks. It acts as a drop-in replacement for the GNU Compiler Collection ...
in 2020. Today, the ClangBuiltLinux
' group coordinates fixes to both
Linux
Linux ( or ) is a family of open-source Unix-like operating systems based on the Linux kernel, an operating system kernel first released on September 17, 1991, by Linus Torvalds. Linux is typically packaged as a Linux distribution, which i ...
and LLVM
LLVM is a set of compiler and toolchain technologies that can be used to develop a front end for any programming language and a back end for any instruction set architecture. LLVM is designed around a language-independent intermediate repre ...
to ensure compatibility, both composed of members from ''LLVMLinux'' and having upstreamed patches from ''LLVMLinux''.
Kernel debugging
Bugs involving the Linux Kernel can be difficult to troubleshoot. This is because of the kernel's interaction with userspace and hardware; and also because they might be caused from a wider range of reasons compared to those of user programs. A few examples of the underlying causes are semantic errors in code, misuse of synchronization primitives, and incorrect hardware management.
A report of a non-fatal bug in the kernel is called an " oops"; such deviations from correct behavior of the Linux kernel may allow continued operation with compromised reliability.
A critical and fatal error is reported via the function. It prints a message and then halts the kernel.
One of the most common techniques used to find out bugs in code is ''debugging by printing''. For this purpose Linux provides an in-kernel API called which stores messages in a circular buffer. The system call is used for reading and/or clearing the kernel message ring buffer and for setting the maximum ''log level'' of the messages to be sent to the console (i.e., one of the eight parameters of , which tell the severity of the condition reported); usually it is invoked via the glibC wrapper . Kernel messages are also exported to userland through the ''/dev/kmsg'' interface (e.g., systemd-journald reads that interface and by default append the messages to ).
Another fundamental technique for debugging a running kernel is tracing. The '' ftrace'' mechanism is a Linux internal tracer; it is used for monitoring and debugging Linux at runtime and it can also analyze user space latencies due to kernel misbehavior. Furthermore, ''ftrace'' allows users to trace Linux at boot-time.
''kprobes'' and ''kretprobes'' can break (like debuggers in userspace) into Linux and non-disruptively collect information. ''kprobes'' can be inserted into code at (almost) any address, while kretprobes work at function return. ''uprobes'' have similar purposes but they also have some differences in usage and implementation.
With KGDB Linux can be debugged in much the same way as userspace programs. KGDB requires an additional machine that runs GDB and that is connected to the target to be debugged using a serial cable or Ethernet
Ethernet () is a family of wired computer networking technologies commonly used in local area networks (LAN), metropolitan area networks (MAN) and wide area networks (WAN). It was commercially introduced in 1980 and first standardized in ...
.
Development model
The Linux kernel project integrates new code on a rolling basis. Software checked into the project must work and compile without error. Each kernel subsystem is assigned a maintainer who is responsible for reviewing patches against the kernel code standards and keeps a queue of patches that can be submitted to Linus Torvalds within a merge window of several weeks. Patches are merged by Torvalds into the source code of the prior stable Linux kernel release, creating the ''-rc'' release candidate for the next stable kernel. Once the merge window is closed only fixes to the new code in the development release are accepted. The ''-rc'' development release of the kernel goes throughregression tests
Regression testing (rarely, ''non-regression testing'') is re-running functional and non-functional tests to ensure that previously developed and tested software still performs as expected after a change. If not, that would be called a '' regres ...
and once it is judged to be stable by Torvalds and the kernel subsystem maintainers a new Linux kernel is released and the development process starts all over again.
Developers who feel treated unfairly can report this to the Linux Foundation
The Linux Foundation (LF) is a non-profit technology consortium founded in 2000 as a merger between Open Source Development Labs and the Free Standards Group to standardize Linux, support its growth, and promote its commercial adoption. Addi ...
's Technical Advisory Board. In July 2013, the maintainer of the USB 3.0 driver Sage Sharp asked Torvalds to address the abusive commentary in the kernel development community. In 2014, Sharp backed out of Linux kernel development, saying that "The focus on technical excellence, in combination with overloaded maintainers, and people with different cultural and social norms, means that Linux kernel maintainers are often blunt, rude, or brutal to get their job done". At the linux.conf.au (LCA) conference in 2018, developers expressed the view that the culture of the community has gotten much better in the past few years. Daniel Vetter, the maintainer of the Intel drm/i915 graphics kernel driver, commented that the "rather violent language and discussion" in the kernel community has decreased or disappeared.
Laurent Pinchart asked developers for feedback on their experience with the kernel community at the 2017 Embedded Linux Conference Europe. The issues brought up were discussed a few days later at the Maintainers Summit. Concerns over the lack of consistency in how maintainers responded to patches submitted by developers were echoed by Shuah Khan
Shuah is the name of one of four minor Biblical figures. It is sometimes used as the name of a fifth. Their names are different in Hebrew, but they were all transliterated as "Shuah" in the King James Version.
Genesis 25
Shuah (Hebrew: שׁוּ ...
, the maintainer of the kernel self-test framework. Torvalds contended that there would never be consistency in the handling of patches because different kernel subsystems have, over time, adopted different development processes. Therefore, it was agreed upon that each kernel subsystem maintainer would document the rules for patch acceptance.
Mainline Linux
The Git tree ofLinus Torvalds
Linus Benedict Torvalds ( , ; born 28 December 1969) is a Finnish software engineer who is the creator and, historically, the lead developer of the Linux kernel, used by Linux distributions and other operating systems such as Android. He also ...
that contains the Linux kernel is referred to as mainline Linux. Every stable kernel release originates from the mainline tree, and is frequently published on kernel.org. Mainline Linux has only solid support for a small subset of the many devices that run Linux. Non-mainline support is provided by independent projects, such as Yocto
A metric prefix is a unit prefix that precedes a basic unit of measure to indicate a multiple or submultiple of the unit. All metric prefixes used today are decadic. Each prefix has a unique symbol that is prepended to any unit symbol. The pre ...
or Linaro, but in many cases the kernel from the device vendor is needed. Using a vendor kernel likely requires a board support package
In embedded systems, a board support package (BSP) is the layer of software containing hardware-specific boot firmware and device drivers and other routines that allow a given embedded operating system, for example a real-time operating system ...
.
Maintaining a kernel tree outside of mainline Linux has proven to be difficult.
''Mainlining'' refers to the effort of adding support for a device to the mainline kernel, while there was formerly only support in a fork or no support at all. This usually includes adding drivers or device tree files. When this is finished, the feature or security fix is considered ''mainlined''.
Linux-like kernel
The maintainer of the stable branch,Greg Kroah-Hartman
Greg Kroah-Hartman (GKH) is a major Linux kernel developer. he is the Linux kernel maintainer for the branch, the staging subsystem, USB, driver core, debugfs, kref, kobject, and the sysfs kernel subsystems, Userspace I/O (with Hans J. Koc ...
, has applied the term ''Linux-like'' to downstream kernel forks by vendors that add millions of lines of code to the mainline kernel. In 2019, Google
Google LLC () is an American Multinational corporation, multinational technology company focusing on Search Engine, search engine technology, online advertising, cloud computing, software, computer software, quantum computing, e-commerce, ar ...
stated that they wanted to use the mainline Linux kernel in Android
Android may refer to:
Science and technology
* Android (robot), a humanoid robot or synthetic organism designed to imitate a human
* Android (operating system), Google's mobile operating system
** Bugdroid, a Google mascot sometimes referred to ...
so the number of kernel forks would be reduced. The term Linux-like has also been applied to the Embeddable Linux Kernel Subset, which does not include the full mainline Linux kernel but a small modified subset of the code.
Linux forks
There are certain communities that develop kernels based on the official Linux. Some interesting bits of code from these forks that include Linux-libre
Linux-libre is a modified version of the Linux kernel that contains no binary blobs, obfuscated code, or code released under proprietary licenses. In the Linux kernel, they are mostly used for proprietary firmware images. While generally re ...
, Compute Node Linux
Compute Node Linux (CNL) is a runtime environment based on the Linux kernel for the Cray XT3, Cray XT4, Cray XT5, Cray XT6, Cray XE6 and Cray XK6 supercomputer systems based on SUSE Linux Enterprise Server. CNL forms part of the Cray Linux E ...
, INK, L4Linux, RTLinux, and User-Mode Linux (UML) have been merged into the mainline. Some operating systems developed for mobile phones initially used heavily modified versions of Linux, including Google Android
Android may refer to:
Science and technology
* Android (robot), a humanoid robot or synthetic organism designed to imitate a human
* Android (operating system), Google's mobile operating system
** Bugdroid, a Google mascot sometimes referred to ...
, Firefox OS
Firefox OS (project name: ''Boot to Gecko'', also known as ''B2G'') is a discontinued open-source operating system made for smartphones, tablet computers, smart TVs, and dongles designed by Mozilla and external contributors. It is based on the ...
, HP webOS
webOS, also known as LG webOS and previously known as Open webOS, HP webOS and Palm webOS, is a Linux kernel-based multitasking operating system for smart devices such as smart TVs that has also been used as a mobile operating system. Initiall ...
, Nokia Maemo
Maemo is a software platform originally developed by Nokia, now developed by the community, for smartphones and Internet tablets. The platform comprises both the Maemo operating system and SDK. Maemo played a key role in Nokia's strategy to c ...
and Jolla Sailfish OS. In 2010, the Linux community criticised Google for effectively starting its own kernel tree:
Today Android uses a customized Linux where major changes are implemented in device drivers, but some changes to the core kernel code is required. Android developers also submit patches to the official Linux that finally can boot the Android operating system. For example, a Nexus 7 can boot and run the mainline Linux.
At a 2001 presentation at the Computer History Museum
The Computer History Museum (CHM) is a museum of computer history, located in Mountain View, California. The museum presents stories and artifacts of Silicon Valley and the information age, and explores the computing revolution and its impact ...
, Linus Torvalds
Linus Benedict Torvalds ( , ; born 28 December 1969) is a Finnish software engineer who is the creator and, historically, the lead developer of the Linux kernel, used by Linux distributions and other operating systems such as Android. He also ...
had this to say in response to a question about distributions of Linux using precisely the same kernel sources or not:
Development community conflicts
There have been several notable conflicts among Linux kernel developers. Examples of such conflicts are: * In July 2007,Con Kolivas
Con Kolivas is an Australian anaesthetist.Anaesthesia Information Page
by Kolivas, Jan 2001 He ...
announced that he would cease developing for the Linux kernel.
* In July 2009, Alan Cox quit his role as the by Kolivas, Jan 2001 He ...
TTY
TTY may refer to:
Communications and technology
* Teleprinter or teletypewriter (TTY), an electromechanical typewriter paired with a communication channel
** Sometimes used more generally for any type of computer terminal
** Sometimes used for a v ...
layer maintainer after disagreement with Linus Torvalds
Linus Benedict Torvalds ( , ; born 28 December 1969) is a Finnish software engineer who is the creator and, historically, the lead developer of the Linux kernel, used by Linux distributions and other operating systems such as Android. He also ...
.
* In December 2010, there was a discussion between Linux SCSI maintainer James Bottomley and SCST maintainer Vladislav Bolkhovitin about which SCSI target stack should be included in the Linux kernel. This made some Linux users upset.
* In June 2012, Torvalds made it very clear that he did not agree with NVIDIA
Nvidia CorporationOfficially written as NVIDIA and stylized in its logo as VIDIA with the lowercase "n" the same height as the uppercase "VIDIA"; formerly stylized as VIDIA with a large italicized lowercase "n" on products from the mid 1990s to ...
releasing its drivers as closed.
* In April 2014, Torvalds banned Kay Sievers from submitting patches to the Linux kernel for failing to deal with bugs
Bugs may refer to:
* Plural of bug
Arts, entertainment and media Fictional characters
* Bugs Bunny, a character
* Bugs Meany, a character in the ''Encyclopedia Brown'' books
Films
* ''Bugs'' (2003 film), a science-fiction-horror film
* ''Bugs ...
that caused systemd to negatively interact with the kernel.
* In October 2014, Lennart Poettering accused Torvalds of tolerating the rough discussion style on Linux kernel related mailing lists and of being a bad role model.
* In March 2015, Christoph Hellwig filed a lawsuit against VMware
VMware, Inc. is an American cloud computing and virtualization technology company with headquarters in Palo Alto, California. VMware was the first commercially successful company to virtualize the x86 architecture.
VMware's desktop software ru ...
for infringement of the copyright on the Linux kernel. Linus Torvalds made it clear that he did not agree with this and similar initiatives by calling lawyers a festering disease.
* In April 2021, a team from the University of Minnesota
The University of Minnesota, formally the University of Minnesota, Twin Cities, (UMN Twin Cities, the U of M, or Minnesota) is a public university, public Land-grant university, land-grant research university in the Minneapolis–Saint Paul, Tw ...
was found to be submitting "bad faith" patches to the kernel as part of their research. This resulted in the immediate reversion of all patches ever submitted by a member of the university. In addition, a warning was issued by a senior maintainer that any future patch from the university would be rejected on sight.
Prominent Linux kernel developers have been aware of the importance of avoiding conflicts between developers. For a long time there was no code of conduct for kernel developers due to opposition by Linus Torvalds
Linus Benedict Torvalds ( , ; born 28 December 1969) is a Finnish software engineer who is the creator and, historically, the lead developer of the Linux kernel, used by Linux distributions and other operating systems such as Android. He also ...
. However, a Linux Kernel ''Code of Conflict'' was introduced on 8 March 2015. It was replaced on 16 September 2018 by a new ''Code of Conduct'' based on the Contributor Covenant. This coincided with a public apology by Torvalds and a brief break from kernel development. On 30 November 2018, complying with the ''Code of Conduct'', Jarkko Sakkinen of Intel sent out patches replacing instances of "fuck" appearing in source code comments with suitable versions focused on the word 'hug'.
Codebase
, the 5.11 release of the Linux kernel had around 30.34 million lines of code. Roughly 14% of the code is part of the "core" (arch, kernel and mm directories), while 60% is drivers.Estimated cost to redevelop
The cost to redevelop version 2.6.0 of the Linux kernel in a traditional proprietary development setting has been estimated to be US$612 million (€467M, £394M) in 2004 prices using the COCOMO person-month estimation model. In 2006, a study funded by the European Union put the redevelopment cost of kernel version 2.6.8 higher, at €882M ($1.14bn, £744M).
This topic was revisited in October 2008 by Amanda McPherson, Brian Proffitt, and Ron Hale-Evans. Using David A. Wheeler's methodology, they estimated redevelopment of the 2.6.25 kernel now costs $1.3bn (part of a total $10.8bn to redevelop Fedora 9). Again, Garcia-Garcia and Alonso de Magdaleno from University of Oviedo (Spain) estimate that the value annually added to kernel was about €100M between 2005 and 2007 and €225M in 2008, it would cost also more than €1bn (about $1.4bn as of February 2010) to develop in the European Union.
, using then-current LOC (lines of code) of a 2.6.x Linux kernel and wage numbers with David A. Wheeler's calculations it would cost approximately $3bn (about €2.2bn) to redevelop the Linux kernel as it keeps getting bigger. An updated calculation , using then-current 20,088,609 LOC (lines of code) for the 4.14.14 Linux kernel and the current US national average programmer salary of $75,506 show it would cost approximately $14,725,449,000 dollars (£11,191,341,000) to rewrite the existing code.
Maintenance and long-term support
The latest kernel version and older kernel versions are maintained separately. Most latest kernel releases were supervised by Linus Torvalds.
The Linux kernel developer community maintains a stable kernel by applying fixes for software bugs that have been discovered during the development of the subsequent stable kernel. Therefore, www.kernel.org will always list two stable kernels. The next stable Linux kernel is now released only 8 to 12 weeks later. Therefore, the Linux kernel maintainers have designated some stable kernel releases as ''longterm'', these long-term support
Long-term support (LTS) is a product lifecycle management policy in which a stable release of computer software is maintained for a longer period of time than the standard edition. The term is typically reserved for open-source software, where it ...
Linux kernels are updated with bug fixes for two or more years. , there are six longterm Linux kernels: 5.15.23, 5.10.100, 5.4.179, 4.19.229, 4.14.266, and 4.9.301. The full list of releases is at Linux kernel version history.
Relation with Linux distributions
Most Linux users run a kernel supplied by theirLinux distribution
A Linux distribution (often abbreviated as distro) is an operating system made from a software collection that includes the Linux kernel and, often, a package management system. Linux users usually obtain their operating system by downloading on ...
. Some distributions ship the "vanilla" or "stable" kernels. However, several Linux distribution vendors (such as Red Hat and Debian
Debian (), also known as Debian GNU/Linux, is a Linux distribution composed of free and open-source software, developed by the community-supported Debian Project, which was established by Ian Murdock on August 16, 1993. The first version of De ...
) maintain another set of Linux kernel branches which are integrated into their products. These are usually updated at a slower pace compared to the "vanilla" branch, and they usually include all fixes from the relevant "stable" branch, but at the same time they can also add support for drivers or features which had not been released in the "vanilla" version the distribution vendor started basing their branch from.
Legal aspects
Licensing terms
Initially, Torvalds released Linux under a license which forbade any commercial use. This was changed in version 0.12 by a switch to theGNU General Public License
The GNU General Public License (GNU GPL or simply GPL) is a series of widely used free software licenses that guarantee end user
In product development, an end user (sometimes end-user) is a person who ultimately uses or is intended to ulti ...
version 2 (GPLv2). This license allows distribution and sale of possibly modified and unmodified versions of Linux but requires that all those copies be released under the same license and be accompanied by - or that, on request, free access is given to - the complete corresponding source code. Torvalds has described licensing Linux under the GPLv2 as the "best thing I ever did".
The Linux kernel is licensed explicitly under GNU General Public License
The GNU General Public License (GNU GPL or simply GPL) is a series of widely used free software licenses that guarantee end user
In product development, an end user (sometimes end-user) is a person who ultimately uses or is intended to ulti ...
version 2 only (GPL-2.0-only) with an explicit syscall exception (Linux-syscall-note), without offering the licensee the option to choose any later version, which is a common GPL extension. Contributed code must be available under GPL-compatible license.
There was considerable debate about how easily the license could be changed to use later GPL versions (including version 3), and whether this change is even desirable. Torvalds himself specifically indicated upon the release of version 2.4.0 that his own code is released only under version 2. However, the terms of the GPL state that if no version is specified, then any version may be used, and Alan Cox pointed out that very few other Linux contributors had specified a particular version of the GPL.
In September 2006, a survey of 29 key kernel programmers indicated that 28 preferred GPLv2 to the then-current GPLv3 draft. Torvalds commented, "I think a number of outsiders... believed that I personally was just the odd man out because I've been so publicly not a huge fan of the GPLv3." This group of high-profile kernel developers, including Torvalds, Greg Kroah-Hartman
Greg Kroah-Hartman (GKH) is a major Linux kernel developer. he is the Linux kernel maintainer for the branch, the staging subsystem, USB, driver core, debugfs, kref, kobject, and the sysfs kernel subsystems, Userspace I/O (with Hans J. Koc ...
and Andrew Morton, commented on mass media about their objections to the GPLv3. They referred to clauses regarding DRM/tivoization
Tivoization is the practice of designing hardware that incorporates software under the terms of a copyleft software license like the GNU General Public License (GNU GPL), but uses hardware restrictions or digital rights management (DRM) to preve ...
, patents, "additional restrictions" and warned a Balkanisation of the "Open Source Universe" by the GPLv3. Linus Torvalds, who decided not to adopt the GPLv3 for the Linux kernel, reiterated his criticism even years later.
Loadable kernel modules
It is debated whether someloadable kernel module
In computing, a loadable kernel module (LKM) is an object file that contains code to extend the running kernel, or so-called ''base kernel'', of an operating system. LKMs are typically used to add support for new hardware (as device drivers) and ...
s (LKMs) are to be considered derivative work
In copyright law, a derivative work is an expressive creation that includes major copyrightable elements of an original, previously created first work (the underlying work). The derivative work becomes a second, separate work independent in fo ...
s under copyright law, and thereby whether or not they fall under the terms of the GPL.
In accordance with the license rules, LKMs using only a public subset of the kernel interfaces are non-derived works, thus Linux gives system administrators the mechanisms to load out-of-tree binary objects into the kernel address space.
There are some out-of-tree loadable modules that make legitimate use of the ''dma_buf'' kernel feature. GPL compliant code can certainly use it. However, a different possible use case would be Nvidia Optimus
Nvidia Optimus is a computer GPU switching technology created by Nvidia which, depending on the resource load generated by client software applications, will seamlessly switch between two graphics adapters within a computer system in order to pro ...
that pairs a fast GPU with an Intel integrated GPU, where the Nvidia GPU writes into the Intel
Intel Corporation is an American multinational corporation and technology company headquartered in Santa Clara, California, Santa Clara, California. It is the world's largest semiconductor chip manufacturer by revenue, and is one of the devel ...
framebuffer when it is active. But, Nvidia cannot use this infrastructure because it necessitates bypassing a rule that can only be used by LKMs that are also GPL. Alan Cox replied on LKML, rejecting a request from one of their engineers to remove this technical enforcement from the API. Torvalds clearly stated on the LKML that " claim that binary-only kernel modules ARE derivative "by default"'".
On the other hand, Torvalds has also said that " negray area in particular is something like a driver that was originally written for another operating system (i.e., clearly not a derived work of Linux in origin). THAT is a gray area, and _that_ is the area where I personally believe that some modules may be considered to not be derived works simply because they weren't designed for Linux and don't depend on any special Linux behaviour". Proprietary graphics drivers, in particular, are heavily discussed.
Whenever proprietary modules are loaded into Linux, the kernel marks itself as being "tainted", and therefore bug reports from tainted kernels will often be ignored by developers.
Firmware binary blobs
The official kernel, that is the Linus git branch at the kernel.org repository, contains binary blobs released under the terms of the GNU GPLv2 license. Linux can also search filesystems to locate binary blobs, proprietary firmware, drivers, or other executable modules, then it can load and link them into kernel space. When it is needed (e.g., for accessing boot devices or for speed) firmware can be built-in to the kernel, this means building the firmware into vmlinux; however this is not always a viable option for technical or legal issues (e.g., it is not permitted to do this with firmware that is non-GPL compatible, although this is quite common nonetheless).Trademark
Linux is a registeredtrademark
A trademark (also written trade mark or trade-mark) is a type of intellectual property consisting of a recognizable sign, design, or expression that identifies products or services from a particular source and distinguishes them from oth ...
of Linus Torvalds
Linus Benedict Torvalds ( , ; born 28 December 1969) is a Finnish software engineer who is the creator and, historically, the lead developer of the Linux kernel, used by Linux distributions and other operating systems such as Android. He also ...
in the United States, the European Union, and some other countries. A legal battle over the trademark began in 1996, when William Della Croce, a lawyer who was never involved in the development of Linux, started requesting licensing fees for the use of the word ''Linux''. After it was proven that the word was in common use long before Della Croce's claimed first use, the trademark was awarded to Torvalds.
See also
* * * * * * * * * * *Notes
References
Further reading
* * * * * * * * * * * * *External links
* ** /kernel.org/doc/ Linux kernel documentation index** /kernel.org/doc/man-pages/ Linux kernel man pages*Kernel bugzilla
an
regressions
for each recent kernel version
Kernel Newbies
a source of various kernel-related information
Kernel coverage at LWN.net
an authoritative source of kernel-related information
Bootlin's Elixir Cross Referencer
a Linux kernel source code cross-reference * {{DEFAULTSORT:Linux Kernel Finnish inventions Free and open-source software Free software programmed in C Free system software Linus Torvalds Monolithic kernels Operating systems Software using the GPL license Unix variants