Software:Linux kernel

From HandWiki
Jump to: navigation, search
Linux kernel
Tux the penguin, mascot of Linux[1]
Linux 3.0.0 boot.png
Linux kernel 3.0.0 booting
DeveloperLinus Torvalds and thousands of collaborators
Written inC and assembly[2]
OS familyUnix-like
Initial release0.02 (5 October 1991; 29 years ago (1991-10-05))
Available inEnglish
Kernel typeMonolithic
LicenseGNU GPLv2 (only) with some code under compatible GPL variants or under permissive licenses like BSD, MIT, etc...[3]

The Linux kernel, developed by contributors worldwide, is a free and open-source, monolithic, modular (i.e., it supports the insertion and removal at runtime of loadable kernel objects[4]), Unix-like operating system kernel.

It is deployed on a wide variety of computing systems, such as embedded devices, mobile devices (including its use in the Android operating system), personal computers, servers, mainframes, and supercomputers.[5]

The Linux kernel was conceived and created in 1991 by Linus Torvalds[6] for his personal computer and with no cross-platform intentions, but has since ported to a wide range of computer architectures. Notwithstanding this, the Linux kernel is highly optimized with the use of architecture specific instructions (ISA),[7] therefore portability isn't as easy as it is with other kernels (e.g., with NetBSD, that as of 2019 supports 59 hardware platforms).

Linux was soon adopted as the kernel for the GNU Operating System,[8] which was created as an open source and free software, and based on UNIX as a by-product of the fallout of the Unix wars.[9] Since then it has spawned a plethora of operating system distributions, commonly also called Linux,[10] although, formally, the term "Linux" refers only to the kernel.[11]

Day-to-day development discussions take place on the Linux kernel mailing list (LKML). Linux as a whole, as it is clearly stated in the COPYING file,[12] is released under the GNU General Public License version 2 (GPLv2), but it also contains several files under other compatible licenses[3] and an ad hoc exemption for the User-space API header files (UAPI).

The Linux ABI (i.e., the Application Binary Interface which also includes Application Program Interface or API at the code source level)[13] between the kernel and the user space has four degrees of stability (stable, testing, obsolete, removed),[14] however the system calls are expected to never change in order to not break the userspace programs that rely on them.[15] As far as in-kernel APIs are regarded, there's no guarantee of stability. Device drivers included in the mainline Linux must be kept updated by their maintainers in order to stay at pace with the kernel evolution. Furthermore, the interface between the kernel and loadable kernel modules (LKMs), unlike in many other kernels, is not meant to be stable by design.[16]


Linus Torvalds

In April 1991, Linus Torvalds, at the time a 21-year-old computer science student at the University of Helsinki, Finland , started working on some simple ideas for an operating system. He started with a 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 on Usenet:[17]

I'm doing a (free) operating system (just a hobby, won't be big and professional like gnu) for 386(486) AT clones. This has been brewing since April, and is starting to get ready. I'd like any feedback on things people like/dislike in minix, as my OS resembles it somewhat (same physical layout of the file-system (due to practical reasons) among other things). I've currently ported bash(1.08) and gcc(1.40), and things seem to work. This implies that I'll get something practical within a few months [...] Yes - it's free of any minix code, and it has a multi-threaded fs. It is NOT portable (uses 386 task switching etc), and it probably never will support anything other than AT-harddisks, as that's all I have :-(.

On 17 September 1991, Torvalds prepared version 0.01 of Linux and put on the "" – 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.[18]

On 5 October 1991, Linus announced the first "official" version of Linux, version 0.02.[19] At this point, Linux was able to run Bash, GCC, and some other GNU utilities:[19][18]

[As] I mentioned a month ago, I'm working on a free version of a Minix-lookalike for AT-386 computers. It has finally reached the stage where it's even usable (though may not be depending on what you want), and I am willing to put out the sources for wider distribution. It is just version 0.02...but I've successfully run bash, gcc, gnu-make, gnu-sed, compress, etc. under it.

After that, many people contributed code to the project, included some developers from the MINIX community. At the time, the GNU Project had created many of the components required for a free operating system, but its own kernel, GNU Hurd, was incomplete and unavailable. The Berkeley Software Distribution had not yet freed itself from legal encumbrances. Despite the limited functionality of the early versions, Linux rapidly gained developers and users.

Torvalds assigned version 0 to the kernel to indicate that it was mainly for testing and not intended for productive use.[20] 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 version 2 (GPLv2) over his previous self-drafted license, which had not permitted commercial redistribution.[21] In contrast to Unix, all source files of Linux are freely available, including device drivers.[22] The initial success of Linux was driven by programmers and testers across the world. With the support of the POSIX 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.[23]

The Linux kernel now supports various hardware architectures, providing a common platform for software, including proprietary software.

On 19 January 1992, the first post to the new newsgroup alt.os.linux was submitted.[24] On 31 March 1992, the newsgroup was renamed comp.os.linux.[25] The fact that Linux is a monolithic kernel rather than a microkernel was the topic of a debate between Andrew S. Tanenbaum, the creator of MINIX, and Torvalds.[26] The Tanenbaum–Torvalds debate started in 1992 on the Usenet group comp.os.minix as a general discussion about kernel architectures.[27][28]

Linux version 0.95 was the first to be capable of running the X Window System.[29] In March 1994, Linux 1.0.0 was released with 176,250 lines of code.[30] It was the first version suitable for use in production environments.[20]

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.[31] 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.[20] 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.[32] 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).[32]

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.[33]

Starting with version 2.0, Linux is configurable for selecting specific hardware targets and for enabling architecture specific features and optimizations.[23] 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.[34][35]

The kernel supports several file system, some that have been designed for Linux, like ext3, ext4, Btrfs,[36] and others that are native of other operating systems like Minix, Xenix, Irix, Solaris, System V, Windows and MS-DOS.[37]

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.[38] In February 2008 the linux-next tree to serve as a place where patches aimed to be merged during the next development cycle gathered.[39][40] Several subsystem maintainers also adopted the suffix -next for trees containing code which they mean to submit for inclusion in the next release cycle. As of January 2014, the in-development version of Linux is held in an unstable branch named linux-next.[41]

Linux used to be maintained without the help of an automated source code management system until, in 2002, development switched to BitKeeper. It was freely available for Linux developers but it was not free software. In 2005, because of efforts to reverse-engineer it, the company which owned the software revoked the 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.[42]

The 20th anniversary of Linux was celebrated by Torvalds in July 2011 with the release of the 3.0.0 kernel version.[33] The Linux Foundation celebrated the 20th anniversary in the 2011 edition of its kernel development study. Kernel 3.0 had 15 million lines of code and over 1,300 individual developers had contributed to this version of the Linux. Volunteer developers contributed 16 percent of the total changes to the kernel in 2011. The other changes were contributions by professional software developers who were paid by a company to submit code to the kernel. As 2.6 has 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.[43]

Stable 3.x.y kernels were released until 3.19 in February 2015, with development releases carrying the -rc designation. To account for the occasional special patch release, the version 3 series of the kernel added a fourth digit to the version numbering. In April 2015, Torvalds released kernel version 4.0.[33] 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.[44] Version 4.1 of Linux, released in June 2015, contains over 19.5 million lines of code contributed by almost 14,000 programmers.[45]


Main page: Software:Usage share of operating systems

According to the Stack Overflow’s annual Developer Survey of 2019, more than the 53% of all respondents have developed software for Linux OS and about 27% for Android,[46] although only about 25% develop with Linux-based operating systems.[47]

Linux distributions that bundle the Linux kernel with system software (eg., the GNU C Library, Systemd, and most of the others Unix utilities and daemons) and application software, as well as the Android operating system, which counts the majority of the installed base of all other operating systems for mobile devices,[48][49][50] are responsible for the rising usage of operating systems based on Linux.[23]

The vast majority of website servers use Linux OS[51][52] and all of the world's 500 most powerful supercomputers use some kind of OS based on Linux.[5] However, the usage share of Linux distributions in desktops is low in comparison to other operating systems.

The Linux kernel is used in a wide variety of embedded devices, such as routers, wireless access points, PBXes, set-top boxes, FTA receivers, smart TVs, PVRs, NAS appliances, and webcams.


Map of the Linux kernel

Linux is a monolithic kernel with a modular design (e.g, it can insert and remove LKM's at runtime), supporting most features once only available in closed source kernels of non-free operating systems:

Device drivers and kernel extensions run in kernel space (ring 0 in many CPU architectures), with full access to the hardware, although some exceptions run in user space, for example, filesystems based on FUSE/CUSE, and parts of UIO.[65][66] The graphics system most people use with Linux does not run within the kernel. 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.[54] By choice, Linux has no stable device driver application binary interface.[16]

Linux typically makes use of memory protection and virtual memory and can also handle non-uniform memory access,[67] however the project has absorbed μClinux which also makes it possible to run Linux on microcontrollers without virtual memory.[68]

The hardware is represented in the file hierarchy. User applications interact with device drivers via entries in the /dev or /sys directories.[69] Processes information as well are mapped to the file system through the /proc directory.[69]

Various layers within Linux, also showing separation between the userland and kernel space
User mode User applications For example, bash, LibreOffice, GIMP, Blender, 0 A.D., Mozilla Firefox, etc.
Low-level system components: System daemons:
systemd, runit, logind, networkd, PulseAudio, ...
Windowing system:
X11, Wayland, SurfaceFlinger (Android)
Other libraries:
GTK+, Qt, EFL, Software:Simple DirectMedia Layer|SDL]], SFML, FLTK,GNUstep|GNUstep]], etc.
Mesa, AMD Catalyst, ...
C standard library open(), exec(), sbrk(), socket(), fopen(), calloc(), ... (up to 2000 subroutines)
glibc aims to be POSIX/SUS-compatible, musl and uClibc target embedded systems, bionic written for Android, etc.
Kernel mode Linux kernel stat, splice, dup, read, open, ioctl, write, mmap, close, exit, etc. (about 380 system calls)
The Linux kernel System Call Interface (SCI, aims to be POSIX/SUS-compatible)
Process scheduling
Memory management
Virtual files
Other components: ALSA, DRI, evdev, Logical Volume Manager (Linux)|LVM]], device mapper, Linux Network Scheduler, Netfilter
Security Modules]]: SELinux, TOMOYO, AppArmor, Smack (Linux security module)|Smack]]
Hardware (CPU, main memory, data storage devices, etc.)

Programming language and coding style

Linux is written in a special C programming language 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.[70][71]

GNU toolchain

The GNU Compiler Collection (GCC or GNU cc) is the default compiler for the mainline Linux sources and it's invoked by a utility called make. Then, the GNU Assembler (more often called GAS or GNU as) outputs the object files from the GCC generated assembly code. Finally, the GNU Linker (GNU ld) is used to produce a statically linked executable kernel file called vmlinux. as and ld are part of a package called GNU binutils. The above-mentioned tools are collectively known as the GNU toolchain.

Compiler compatibility

GCC was for a long time the only compiler capable of correctly building Linux. In 2004, Intel claimed to have modified the kernel so that its C compiler was also capable of compiling it.[72] There was another such reported success in 2009, with a modified 2.6.22 version.[73][74]

Since 2010, effort has been underway to build Linux with Clang, an alternative compiler for the C language;[75] as of 12 April 2014, the official kernel could almost be compiled by Clang.[76][77] The project dedicated to this effort is named LLVMLinux after the LLVM compiler infrastructure upon which Clang is built.[78] 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 that, among other advantages, kernel developers may benefit from shorter compilation times.[79]


Four interfaces are distinguished: two internal to the kernel, and two between the kernel and userspace.

Linux is a clone of UNIX, therefore it aims towards POSIX and Single UNIX Specification compliance.[80]

Furthermore, since Linux is not UNIX, the kernel provides additional 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 well defined licensing rules.[12][3]

Kernel-to-userspace API

The set of the Linux kernel API that regard the interfaces exposed to user applications is fundamentally composed by UNIX and Linux-specific system calls.[81] A system call is an entry point into the Linux kernel.[82] For example, among the Linux-specific ones there is the family of the clone() system calls.[83] Most extensions must be enabled by defining the _GNU_SOURCE macro in a header file or when the user-land code is being compiled.[84]

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,[85] can transparently enter into the kernel which will execute on behalf of the calling process.[81] For those system calls not exposed by libC, e.g. the fast userspace mutex (futex),[86] the library provides a function called syscall() which can be used to explicitly invoke them.[87]

Pseudo filesystems (e.g., the sysfs and procfs filesystems) and special files (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.[88][89]

Kernel-to-userspace ABI

Main page: Linux Standard BaseBecause 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 the ISA of the target hardware), often cannot run on different Linux Distributions. This issue is mainly due to distribution-specific configurations and 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).[90][91] 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.[92] The LSB version 5 is built upon several standards and drafts (POSIX, SUS, X/Open, File System Hierarchy (FHS), and others).[93]

The parts of the LSB largely relevant to the kernel are the General ABI (gABI),[94] especially the System V ABI[95][96] and the Executable and Linkable Format (ELF),[97] and the Processor Specific ABI (psABI), for example the Core Specification for X86-64.[98][99]

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.[100][101][102]

In-kernel API

At XDC2014, Alex Deucher from AMD announced the unified kernel-mode driver.[103] The proprietary Linux graphic driver, libGL-fglrx-glx, will share the same DRM infrastructure with Mesa 3D. As there is no stable in-kernel ABI, AMD had to constantly adapt the former binary blob used by Catalyst.

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 EXPORT_SYMBOL() and EXPORT_SYMBOL_GPL() macros[104][105] (the latter reserved to modules released under a GPL-compatible license).[106]

Some Linux in-kernel APIs have been kept stable over several releases, others have not. There are no guarantees regarding the in-kernel APIs. Maintainers and contributors are free to augment or change them at any time.[16] However, there are in-kernel APIs that manipulate data structures (e.g., lists, trees, 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.[107][108][109]

Examples of in-kernel APIs include libraries of low-level common services used by device drivers:

  • SCSI 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,[110] and an in-kernel library to support [S]ATA host controllers and devices.[111]
  • Direct Rendering Manager (DRM) and Kernel Mode Setting (KMS) - for interfacing with GPUs and supporting the needs of modern 3D-accelerated video hardware,[112] and for setting screen resolution, color depth and refresh rate[113]
  • DMA buffers (dma buf) - for sharing buffers for hardware direct memory access across multiple device drivers and subsystems[114][115][116]
  • Video4Linux – for video capture hardware
  • Advanced Linux Sound Architecture (ALSA) – for sound cards
  • New API – for network interface controllers
  • mac80211 – for wireless network interface controllers[117]

In-kernel ABI

The Linux developers choose not to maintain a stable in-kernel ABI.[118]

Technical features

The Linux Storage Stack Diagram[119]

Processes and Threads

See also: Process (computing), Thread_(computing), and Process_management (computing)Linux creates processes by means of the clone() or by the newer clone3()[120] 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, if the CLONE_THREAD parameter is given to the above mentioned syscalls. In this latter case the new entity owns the same TGID of the calling process (and consequently has also the same PID).[121][122]

If a new thread within the same process is created, in conformity with the POSIX threads requirements, the clone() family of system calls must also be given the address of the function that the new thread must jump to.

If a new independent processes is created, the syscalls return exactly to the next instruction of the same program, but concurrently in the parent and in the child processes (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 execve() system call (possibly via the family of exec() 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.[123] When it's done, it results in two processes that run two different programs.

Scheduling and Preemption

The Linux scheduler enables different classes and policies.[124] By default the kernel uses a scheduler mechanism called the Completely Fair Scheduler introduced in the 2.6.23 version of the kernel.[55] Internally this default-scheduler class is also known as SCHED_OTHER, but the kernel also contains two POSIX-compliant[125] 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.[124] 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.[126][127] SCHED_DEADLINE takes precedence over all the other scheduling classes.

Linux provides both user preemption as well as full kernel preemption.[128] Preemption reduces latency, increases responsiveness, and makes Linux more suitable for desktop and real-time applications.

With user preemption, the kernel scheduler can replace the current process with the execution of a context switch to a different one that therefore acquires the computing resources for running (CPU, memory, and more). It makes it according to the CFS algorithm (in particular it uses a variable called vruntime for sorting processes), to the active scheduler policy and to the processes 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.

Through the use of the real-time Linux kernel patch PREEMPT_RT, support for full preemption of critical sections, interrupt handlers, and "interrupt disable" code sequences can be supported.[129] Partial mainline integration of the real-time Linux patches already brought some functionality to the kernel mainline.[130]

Memory and Address Spaces

Inter Process Communication and Synchronization

Input/Output Layer

Virtual and Concrete Filesystems

Device Drivers


TiVo DVR, a consumer device running Linux

While not originally designed to be portable,[17][131] Linux is now one of the most widely ported operating system kernels, running on a diverse range of systems from the ARM architecture to IBM z/Architecture mainframe computers. The first port beyond Linux's original 386 architecture was performed on the Motorola 68000 platform by Amiga users, who accomplished this by replacing major parts of the kernel. The modifications to the kernel were so fundamental that Torvalds viewed the Motorola version as a fork and a "Linux-like operating system"[131] rather than as an actual port. It was, however, the impetus that Torvalds needed to lead a major restructure of the kernel code to facilitate porting to competing computing architectures. The first Linux endorsed port was to the DEC Alpha AXP 64-bit platform which was demonstrated at DECUS in May 1995,[132] supporting both 386 and Alpha in a single source tree.[131] DEC was responsible for supplying the hardware necessary to Torvalds to enable a port of Linux to 64 bits[133] that same year.

Linux runs as the main operating system on IBM's Summit and all other fastest supercomputers, including the Chinese-designed and built Sunway TaihuLight (formerly fastest); as of October 2019, all of the world's 500 fastest supercomputers run some variant of Linux,[5] a big change from 1998 when the first Linux supercomputer got added to the list then ranked 113.[134] Unix had dominated the list previously; the list hit a roughly even split between Unix and Linux in about 2003.

Linux has also been ported to various handheld devices such as Apple's iPhone 3G and iPod.[135]

Linux forks
An iPod booting iPodLinux

There are certain communities that develop kernels based on the official Linux. Some interesting bits of code from these forks (i.e., a slang term meaning "derived projects") that include Linux-libre, Compute Node Linux, Cooperative Linux, Longene, grsecurity, INK, L4Linux, MkLinux, RTLinux, and User-Mode Linux (UML) have been merged into the mainline.[136] Some operating systems developed for mobile phones initially used heavily modified versions of Linux, including Google Android, Firefox OS, HP webOS, Nokia Maemo and Jolla Sailfish OS. In 2010, the Linux community criticised Google for effectively starting its own kernel tree:[137][138]

This means that any drivers written for Android hardware platforms, can not get merged into the main kernel tree because they have dependencies on code that only lives in Google's kernel tree, causing it to fail to build in the tree. Because of this, Google has now prevented a large chunk of hardware drivers and platform code from ever getting merged into the main kernel tree. Effectively creating a kernel branch that a number of different vendors are now relying on.[139]
—Greg Kroah-Hartman, 2010

Today Android uses a slightly customized Linux[140] where changes are implemented in device drivers so that little or no change 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.[140]

Kernel panic and oopses

An example of Linux kernel panic
Main pages: Software:Kernel panic and Linux kernel oops

In Linux, a "panic" is an unrecoverable system error detected by the kernel which signals such a condition by calling the panic function located in the header filesys/system.h. Most panics are the result of unhandled processor exceptions, such as references to invalid memory addresses. They are typically indicative of a bug in kernel code. Others can also indicate a failure of hardware, caused by a processor bug, overheating/damaged CPU, RAM, and motherboard soft errors.

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.[141]

Live patching

Rebootless updates can even be applied to the kernel by using live patching 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 (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. As of April 2015, there is ongoing work on porting kpatch and kGraft to the common live patching core provided by the Linux kernel mainline.[142][143][144]


Computer security is a much-publicized topic in relation to the Linux kernel because a large portion of the kernel bugs present potential security flaws. For example, they may allow for privilege escalation or create denial-of-service attack vectors. Over the years, numerous such flaws were found and fixed in the Linux kernel.[145] New security features are frequently implemented to improve the Linux kernel's security.[146][147]

Linux offers a wealth of mechanisms to reduce kernel attack surface and improve security which are collectively known as the Linux Security Modules (LSM).[148] They comprise the Security-Enhanced Linux (SELinux) module, whose code has been originally developed and then released to the public by the NSA,[149] and AppArmor[64] among others. SELinux is now actively developed and maintained on GitHub.[63] 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.[150]

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:[151][152]

I personally consider security bugs to be just "normal bugs". I don't cover them up, but I also don't have any reason what-so-ever to think it's a good idea to track them and announce them as something reason I refuse to bother with the whole security circus is that I think it glorifies—and thus encourages—the wrong behavior. It makes "heroes" out of security people, as if the people who don't just fix normal bugs aren't as important. In fact, all the boring normal bugs are way more important, just because there's a lot more of them. I don't think some spectacular security hole should be glorified or cared about as being any more "special" than a random spectacular crash due to bad locking.

Linux distributions typically release security updates to fix vulnerabilities in the Linux kernel. Many offer long-term support releases that receive security updates for a certain Linux kernel version for an extended period of time.

Feature history

Version 1.0 of the Linux kernel was released on 14 March 1994.[153] This release of the Linux kernel only supported single-processor i386-based computer systems. Portability became a concern, and so version 1.2 (released 7 March 1995)[154] gained support for computer systems using processors based on the Alpha, SPARC, and MIPS architectures.

Version 2.0 was released on 9 June 1996.[155] The series included 41 releases. The major feature of 2.0 was support for symmetric multiprocessing (SMP) and support for more types of processors.

Version 2.2, released on 20 January 1999,[156] removed the global spinlock and provided improved SMP support, added support for the m68k and PowerPC architectures, and added new file systems (including read-only support for Microsoft's NTFS).[157] In 1999, IBM published its patches to the Linux 2.2.13 code for the support of the S/390 architecture.[158]

Version 2.4.0, released on 4 January 2001,[159] contained support for ISA Plug and Play, USB, and PC Cards.[160] It also included support for the PA-RISC processor from Hewlett-Packard. 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, Logical Volume Manager (LVM) version 1, RAID support, InterMezzo and ext3 file systems.

Version 2.6.0 was released on 17 December 2003.[161] 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),[162] substantially increased the number of device types and the number of devices of each type, improved 64-bit support, support for file systems which support file sizes of up to 16 terabytes, in-kernel preemption, support for the Native POSIX Thread Library (NPTL), User-mode Linux integration into the mainline kernel sources, SELinux integration into the mainline kernel sources, InfiniBand support, and considerably more. Also notable are the addition of several file systems throughout the 2.6.x releases: FUSE, JFS, XFS, ext4 and more. 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[163]

Version 3.0 was released on 22 July 2011.[164] 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."[165] 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 processors, making the 3.7 kernel series the last one still supporting the original processor.[166][167] The same series unified support for the ARM processor.[168]

Version 3.11, released on 2 September 2013,[169] adds many new features such as new O_TMPFILE flag for open(2) to reduce temporary file vulnerabilities, experimental AMD Radeon dynamic power management, low-latency network polling, and zswap (compressed swap cache).[170]

Version 5.0, released in 2019,[171] includes patches for the Spectre and Meltdown security vulnerabilities and other feature enhancements.[172]

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.[164][173]


<graph>Script error: No such module "Graph".</graph>

Developer community

It is generally assumed that the community of Linux kernel developers comprises 5000 or 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. As of 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.[174]

Instead of a roadmap, there are technical guidelines. Instead of a central resource allocation, there are persons and companies who all have a stake in the further development of the Linux kernel, quite independently from one another: People like Linus Torvalds and I don’t plan the kernel evolution. We don’t sit there and think up the roadmap for the next two years, then assign resources to the various new features. That's because we don’t have any resources. The resources are all owned by the various corporations who use and contribute to Linux, as well as by the various independent contributors out there. It's those people who own the resources who decide...
—Andrew Morton, 2005

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 of source code. 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.[175] 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.[176]

Development model

The Linux kernel project integrates new code on a rolling basis. Software checked into the project must work and compile without error. For each kernel subsystem there is 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 through regression tests 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.[177]

Developers who feel treated unfairly can report this to the Linux Foundation's Technical Advisory Board.[178] In July 2013 the maintainer of the USB 3.0 driver Sarah 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".[179] At the (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.[180]

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 a few days later discussed at the Maintainers Summit. Concerns over the lack of consistency in how maintainers responded to patches submitted by developers were echoed by Shuah Khan, 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.[181]

Development community conflicts

There have been several notable conflicts among Linux kernel developers. Examples of such conflicts are:

  • In July 2007, Con Kolivas announced that he would cease developing for the Linux kernel.[182][183]
  • In July 2009, Alan Cox quit his role as the TTY layer maintainer after disagreement with Linus Torvalds.[184]
  • 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.[185] This made some Linux users upset.[186]
  • In June 2012, Torvalds made it very clear that he did not agree with NVIDIA releasing its drivers as closed.[187]
  • In April 2014, Torvalds banned Kay Sievers from submitting patches to the Linux kernel for failing to deal with bugs that caused systemd to negatively interact with the kernel.[188]
  • 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.[189]
  • In March 2015, Christoph Hellwig filed a lawsuit against VMware for infringement of the copyright on the Linux kernel.[190] Linus Torvalds made it clear that he did not agree with this and similar initiatives by calling lawyers a festering disease.[191]

Prominent Linux kernel developers have been aware of the importance of avoiding conflicts between developers.[192] For a long time there has been no code of conduct for kernel developers due to opposition by Linus Torvalds.[193] However, a Linux Kernel Code of Conflict was introduced on 8 March 2015.[194] 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 Linus and a brief break from kernel development.[195][196] 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'.[197]


As of 2020, the 5.6 release of the Linux kernel had around 33 million lines of code, roughly 14% of the code is part of the "core" (arch, kernel and mm directories) while 60% is drivers.

Linux is evolution, not intelligent design!
—Linus Torvalds, 2005[198][199][200]

Estimated cost to redevelop

Redevelopment costs of Linux kernel

The cost to redevelop the Linux kernel version 2.6.0 in a traditional proprietary development setting has been estimated to be US$612 million (€467M, £394M) in 2004 prices using the COCOMO man-month estimation model.[201] 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).[202]

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).[203] 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.[204]

As of 7 March 2011, 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 as of 26 September 2018, 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 pounds) to rewrite the existing code.[205]

Maintenance and long-term support

Boot messages of a Linux kernel

The latest kernel version and older kernel versions are maintained separately. Most latest kernel releases were supervised by Linus Torvalds.[206] Current versions are released by Greg Kroah-Hartman.[207]

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, 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 Linux kernels are updated with bug fixes for two or more years.[208] In November 2019 there were five longterm Linux kernels: 4.19.84, 4.14.154, 4.9.201, 4.4.201 and 3.16.76.[209] The full list of releases is at Linux kernel version history.

Relation with Linux distributions

Most Linux users run a kernel supplied by their Linux distribution. Some distributions ship the "vanilla" or "stable" kernels. However, several Linux distribution vendors (such as Red Hat and Debian) 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.

Source code management

The Linux kernel development community uses Git to manage the kernel source code. Linus Torvalds initially developed this version control system with speed in mind and as a distributed system. Git users can obtain the latest pushed version of Torvalds' tree and keep up to date with the official kernel tree using the git pull.[210] The kernel source code is distributed in GNU zip (gzip) and bzip2 format. Source code contributions by developers are submitted as patches and incremental changes to the kernel source code means developers can seamlessly move from one Linux kernel version to the next.[211]

Legal aspects

GPLv2 licensing terms

Initially, Torvalds released Linux under a license which forbade any commercial use.[212] This was changed in version 0.12 by a switch to the GNU General Public License version 2 (GPLv2).[21] 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 the complete corresponding source code.[213] Torvalds has described licensing Linux under the GPLv2 as the "best thing I ever did".[212]

The Linux kernel is licensed explicitly only under version 2 of the GPL,[12] without offering the licensee the option to choose "any later version", which is a common GPL extension. 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.[214] Torvalds himself specifically indicated upon the release of version 2.4.0 that his own code is released only under version 2.[215] However, the terms of the GPL state that if no version is specified, then any version may be used,[216] and Alan Cox pointed out that very few other Linux contributors had specified a particular version of the GPL.[217]

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."[218] This group of high-profile kernel developers, including Torvalds, Greg Kroah-Hartman and Andrew Morton, commented on mass media about their objections to the GPLv3.[219] They referred to clauses regarding DRM/tivoization, patents, "additional restrictions" and warned a Balkanisation of the "Open Source Universe" by the GPLv3.[219][220] Linus Torvalds, who decided not to adopt the GPLv3 for the Linux kernel, reiterated his criticism even years later.[221]

Loadable kernel modules

It is debated whether loadable kernel modules (LKMs) are to be considered derivative works under copyright law, and thereby fall under the terms of the GPL.

Torvalds has stated his belief that LKMs using only a limited, "public" subset of the kernel interfaces can sometimes be non-derived works, thus allowing some binary-only drivers and other LKMs that are not licensed under the GPL.[citation needed] A good example for this is the usage of dma_buf by the proprietary Nvidia graphics drivers. dma_buf is a recent[when?] kernel feature (like the rest of the kernel, it is licensed under the GPL), which allows multiple GPUs to quickly copy data into each other's framebuffers.[222] One possible use case would be Nvidia Optimus that pairs a fast GPU with an Intel integrated GPU, where the Nvidia GPU writes into the Intel framebuffer when it is active. But, Nvidia cannot use this infrastructure because it uses a technical means to enforce the rule that it 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.[223] Not all Linux kernel contributors agree with this interpretation, however, and even Torvalds agrees that many LKMs are clearly derived works, and indeed he writes that "kernel modules ARE derivative 'by default'".[224]

On the other hand, Torvalds has also said that "one gray 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".[225] Proprietary graphics drivers, in particular, are heavily discussed. Ultimately, it is likely that such questions can only be resolved by a court.

Firmware binary blobs

The official kernel, that is the Linus git branch at the repository, doesn't contain any kind of proprietary code;[12][3] however Linux can search the filesystems to locate proprietary firmware, drivers, and other executable modules (collectively known as "binary blobs"), then it can load and link them into the kernel space.[226] Whenever proprietary modules are loaded into Linux, the kernel marks itself as being "tainted"[227] and therefore bug reports from tainted kernels will often be ignored by developers.

Whether it's 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's not permitted to firmware that is non-GPL compatible).[228]


Linux is a registered trademark of Linus Torvalds in the United States, the European Union, and some other countries.[229][230] This is the result of an incident in which William Della Croce, Jr., who was not involved in the Linux project, trademarked the name and subsequently demanded royalties for its use.[231] Several Linux backers retained legal counsel and filed suit against Della Croce. The issue was settled in August 1997 when the trademark was assigned to Linus Torvalds.[232][233]

See also


  1. "Linux Logos and Mascots". Linux Online. 2008. Archived from the original on 15 August 2010. Retrieved 11 August 2009. 
  2. The Linux Kernel Open Source Project on Open Hub: Languages Page
  3. 3.0 3.1 3.2 3.3 "Linux kernel licensing rules — The Linux Kernel documentation". 
  4. Love, Robert (2010) (in en). Linux kernel development. Addison-Wesley. p. 338. ISBN 978-0-672-32946-3. OCLC 268788260. 
  5. 5.0 5.1 5.2 "TOP500 Supercomputer Sites: Operating system Family / Linux". Retrieved 5 October 2019. 
  6. Richardson, Marjorie (1 November 1999). "Interview: Linus Torvalds". Linux Journal. Retrieved 20 August 2009. 
  7. Love, Robert (2010). Linux Kernel Development. USA: Addison Wesley. pp. 379–380. ISBN 9780672329463. 
  8. Williams, Sam (March 2002). "Chapter 9: The GNU General Public License". Free as in Freedom: Richard Stallman's Crusade for Free Software. O'Reilly. ISBN 0-596-00287-4. Retrieved 12 November 2010. 
  9. Unix System Laboratories v. Berkeley Software, 832 F. Supp. 790 (D.N.J. 1993).
  10. "README".;a=blob;f=README;h=90a07658ede14840346eee6610648bcf4ec79997;hb=f3b8436ad9a8ad36b3c9fa1fe030c7f38e5d3d0b. 
  11. Love, Robert (2010) (in en). Linux kernel development. Addison-Wesley. p. 4. ISBN 978-0-672-32946-3. OCLC 268788260. 
  12. 12.0 12.1 12.2 12.3 "Linux source code: COPYING (v5.4.8) - Bootlin". 
  13. "Binary Compatibility" (in en). 
  14. "README\ABI\Documentation - kernel/git/torvalds/linux.git - Linux kernel source tree". 
  15. "syscalls\stable\ABI\Documentation - kernel/git/torvalds/linux.git - Linux kernel source tree". 
  16. 16.0 16.1 16.2 "stable-api-nonsense - Linux kernel source tree". 
  17. 17.0 17.1 Torvalds, Linus Benedict (26 August 1991). "What would you like to see most in minix?". Newsgroupcomp.os.minix. Usenet: 1991Aug25.205708.9541@klaava.Helsinki.FI.
  18. 18.0 18.1 Welsh, Matt; Dalheimer, Matthias Kalle; Kaufman, Lar (1999). "1". Running Linux (3rd ed.). Sebastopol, CA: O'Reilly Media, Inc.. ISBN 1-56592-976-4. OCLC 50638246. 
  19. 19.0 19.1 "Free minix-like kernel sources for 386-AT - Google Groups". 1991-10-05.!topic/comp.os.minix/4995SivOl9o. 
  20. 20.0 20.1 20.2 Christine Bresnahan & Richard Blum (2016). LPIC-2: Linux Professional Institute Certification Study Guide: Exam 201 and Exam 202. John Wiley & Sons. p. 107. ISBN 9781119150794. 
  21. 21.0 21.1 Torvalds, Linus. "Release Notes for Linux v0.12". The Linux Kernel Archives. Retrieved 21 February 2007. 
  22. Fred Hantelmann (2016). LINUX Start-up Guide: A self-contained introduction. Springer Science & Business Media. p. 1. ISBN 9783642607493. 
  23. 23.0 23.1 23.2 Fred Hantelmann (2016). LINUX Start-up Guide: A self-contained introduction. Springer Science & Business Media. p. 16. ISBN 9783642607493. 
  24. Summers, David W. (19 January 1992). "Troubles with Partitions". Newsgroupalt.os.linux. Usenet: Retrieved 7 January 2007.
  25. Clegg, Alan B. (31 March 1992). "It's here!". Newsgroupcomp.os.linux. Usenet: Retrieved 7 January 2007.
  26. "Appendix A: The Tanenbaum-Torvalds Debate". Open Sources: Voices from the Open Source Revolution. O'Reilly. 1999. ISBN 1-56592-582-3. Retrieved 22 November 2006. 
  27. Tanenbaum, Andy (29 January 1992). "LINUX is obsolete". Newsgroupcomp.os.minix. Usenet: Retrieved 10 May 2006.
  28. Tanenbaum, Andy (12 May 2006). "Tanenbaum-Torvalds Debate: Part II". VU University Amsterdam. Retrieved 6 January 2007. 
  29. November 2012, David Hayward22. "The history of Linux: how time has shaped the penguin" (in en). 
  30. November 2012, David Hayward22. "The history of Linux: how time has shaped the penguin" (in en). 
  31. Love, Robert (Robert M.) (2010). Linux kernel development (3rd ed.). Upper Saddle River, NJ: Addison-Wesley. pp. 9. ISBN 978-0-672-32946-3. OCLC 268788260. 
  32. 32.0 32.1 "How the development process works — The Linux Kernel documentation". 
  33. 33.0 33.1 33.2 Christine Bresnahan & Richard Blum (2016). LPIC-2: Linux Professional Institute Certification Study Guide: Exam 201 and Exam 202. John Wiley & Sons. p. 108. ISBN 9781119150794. 
  34. "Kernel Build System — The Linux Kernel documentation". 
  35. "Kconfig make config — The Linux Kernel documentation". 
  36. "btrfs Wiki". 
  37. Fred Hantelmann (2016). LINUX Start-up Guide: A self-contained introduction. Springer Science & Business Media. pp. 1–2. ISBN 9783642607493. 
  38. Kroah-Hartman, Greg (3 August 2006). "Adrian Bunk is now taking over the 2.6.16-stable branch". LKML (Mailing list). Retrieved 21 February 2015.
  39. Rothwell, Stephen (12 February 2008). "Announce: Linux-next (Or Andrew's dream :-))". LKML (Mailing list). Retrieved 30 October 2010.
  40. Corbet, Jonathan (21 October 2010). "linux-next and patch management process". Eklektix, Inc. Retrieved 30 October 2010. 
  41. "The Linux Kernel Archives". Retrieved 22 January 2014. 
  42. Linux Kernel Mailing List (17 June 2005). "Linux 2.6.12". git-commits-head (Mailing list).
  43. "Add a personality to report 2.6.x version numbers ["]. 
  44. "The Linux Foundation Releases Linux Development Report". Linux Foundation. 18 February 2015. Archived from the original on 19 July 2016. Retrieved 20 February 2015. 
  45. Michael Larabel (23 June 2014). "Linux Kernel At 19.5 Million Lines Of Code, Continues Rising". Phoronix. Retrieved 23 June 2015. 
  46. "Stack Overflow Developer Survey 2019 - most popular technologies". 
  47. "Stack Overflow Developer Survey 2019 - development environments and tools". 
  48. "Gartner Says Sales of Tablets Will Represent Less Than 10 Percent of All Devices in 2014" (Press release). Egham, UK: Gartner. 15 October 2014. Retrieved 19 October 2014.
  49. Lunden, Ingrid (15 October 2014). "Tablet Sales Growth Plummets In 2014 As Android Smartphones Continue To Soar: Gartner". TechCrunch. AOL. Retrieved 23 October 2014. 
  50. "Global PC Shipments Exceed Forecast with Mild Improvement in Consumer Demand, While Apple Moves to #5 Spot, According to IDC" (Press release). Framingham, MA: IDC. 8 October 2014. Archived from the original on 11 October 2014. Retrieved 19 October 2014.
  51. "Usage Statistics and Market Share of Operating Systems for Websites, March 2020". 
  52. "Usage Statistics and Market Share of Unix for Websites, March 2020". 
  53. "FAQ: Preemption". 22 August 2009. Retrieved 7 May 2015. 
  54. 54.0 54.1 Jonathan Corbet (24 February 2003). "Driver porting: the preemptible kernel". Retrieved 7 May 2015. 
  55. 55.0 55.1 Molnár, Ingo (13 April 2007). "[patch] Modular Scheduler Core and Completely Fair Scheduler [CFS]". LKML (Mailing list). Retrieved 30 March 2020.
  56. "Completely Fair Scheduler | Linux Journal". 
  57. "KVM". 
  58. "TechComparison - Linux Virtualization Wiki". 
  59. "Virtualization_support_through_KVM in Linux_2_6_20 - Linux Kernel Newbies". 
  60. Coekaerts, Wim. "Linux mainline contains all the Xen code bits for Dom0 and DomU support". 
  61. "Xen celebrates full Dom0 and DomU support in Linux 3.0 –". 2011-06-07. 
  62. Wilk, Konrad Rzeszutek (2014-01-31). "Linux 3.14 and PVH" (in en-US). 
  63. 63.0 63.1 "SELinux Project" (in en). 
  64. 64.0 64.1 "AppArmor — The Linux Kernel documentation". 
  65. Jake Edge (25 November 2008). "Character devices in user space". Retrieved 7 May 2015. 
  66. Jonathan Corbet (2 May 2007). "UIO: user-space drivers". Retrieved 7 May 2015. 
  67. Gorman, Mel (2004-02-15). Understanding the Linux Virtual Memory Manager. Prentice Hall. p. 26. ISBN 0-13-145348-3. 
  68. Greg Ungerer. "uClinux mainline Announcement". Archived from the original on 31 October 2007. Retrieved 2008-01-15. 
  69. 69.0 69.1 Nguyen, Binh (30 July 2004). "Linux Filesystem Hierarchy: Chapter 1. Linux Filesystem Hierarchy". The Linux Documentation Project. Retrieved 28 November 2012. 
  70. "HOWTO do Linux kernel development — The Linux Kernel documentation". 
  71. "Linux kernel coding style — The Linux Kernel documentation". 
  72. Kubbilun, Ingo A. (2 June 2004). "Linux kernel patch for Intel Compiler" (in de). Archived from the original on 22 July 2011. Retrieved 12 November 2010. 
  73. timothy (26 February 2009). "High Performance Linux Kernel Project — LinuxDNA". Slashdot Linux. Dice Holdings. Retrieved 30 October 2010. 
  74. Ryan, Justin (25 February 2009). "LinuxDNA Supercharges Linux with the Intel C/C++ Compiler". Linux Journal. Belltown Media, Inc.. Retrieved 30 October 2010. 
  75. Lelbach, Bryce (25 October 2010). "Clang builds a working Linux Kernel (Boots to RL5 with SMP, networking and X, self hosts)". cfe-dev (Mailing list). Archived from the original on 7 September 2015.
  76. Larabel, Michael (12 April 2014). "Linux 3.15 Can Almost Be Compiled Under LLVM's Clang". Phoronix. Retrieved 10 June 2014. 
  77. Larabel, Michael. "Patch By Patch, LLVM Clang Gets Better At Building The Linux Kernel". Phoronix. Retrieved 20 November 2014. 
  78. Edge, Jake (7 May 2013). "LFCS: The LLVMLinux project". Retrieved 3 March 2015. 
  79. Möller, Jan-Simon (2 February 2014). "LLVMLinux: The Linux Kernel with Dragon Wings". LLVM Project. Retrieved 3 March 2015. 
  80. "Linux kernel release 5.x — The Linux Kernel documentation". 
  81. 81.0 81.1 "syscalls". 
  82. "intro(2) - Linux manual page". 
  83. "clone". 
  84. "feature_test_macros". 
  85. "vdso(7) - Linux manual page". 
  86. "futex(2) - Linux manual page". 
  87. "syscall(2) - Linux manual page". 
  88. "sysfs(5) - Linux manual page". 
  89. "Rules on how to access information in sysfs — The Linux Kernel documentation". 
  90. "Linux Foundation Referenced Specifications". 
  91. "LSB Specifications". 
  92. "Linux Standard Base Desktop Specification, Generic Part". 
  93. "Normative References". 
  94. "Linux Standard Base Core Specification, Generic Part". 
  95. "System V Application Binary Interface - Edition 4.1". 
  96. "Xinuos Inc. | Developers | Gabi | 2003-12-17 | System V Application Binary Interface - DRAFT". 
  97. "Executable And Linking Format (ELF)". 
  98. "Linux Standard Base Core Specification for X86-64". 
  99. "System V Application Binary Interface - DRAFT". 
  100. Seyfarth, Ray (2012). Introduction to 64 Bit Intel Assembly Language Programming for Linux. pp. 170. ISBN 9781478119203. 
  101. "Anatomy of a system call, part 1 ["]. 
  102. "Anatomy of a system call, part 2 ["]. 
  103. Deucher, Alex (7 October 2014). "AMD's New Unified Open Source Driver". X.Org Foundation. Retrieved 21 January 2015. 
  104. "Symbols - Unreliable Guide To Hacking The Linux Kernel — The Linux Kernel documentation". 
  105. "Exported symbols and the internal API ["]. 
  106. "Unexporting kallsyms_lookup_name() ["]. 
  107. "Unreliable Guide To Hacking The Linux Kernel". 2005. 
  108. "Unreliable Guide To Hacking The Linux Kernel — The Linux Kernel documentation". 
  109. "Unreliable Guide To Locking — The Linux Kernel documentation". 
  110. "SCSI Interfaces Guide — The Linux Kernel documentation". 
  111. "libATA Developer’s Guide — The Linux Kernel documentation". 
  112. "DRM Internals — The Linux Kernel documentation". 
  113. "Kernel Mode Setting (KMS) — The Linux Kernel documentation". 
  114. "Introduce DMA buffer sharing mechanism ["]. 
  115. "Sharing CPU and GPU buffers on Linux*". 2016-05-12. 
  116. "Buffer Sharing and Synchronization — The Linux Kernel documentation". 
  117. "About mac80211". Linux Kernel Organization, Inc.. Retrieved 8 June 2014. 
  118. "Report on ABI changes in the Linux kernel". Andrey Ponomarenko's ABI laboratory. 17 March 2016. 
  119. "The Linux Storage Stack Diagram". 
  120. "[PATCH v3 1/2 fork: add clone3 []"]. 
  121. "clone(2) - Linux manual page". 
  122. "clone3(), fchmodat4(), and fsinfo() ["]. 
  123. "execve(2) - Linux manual page". 
  124. 124.0 124.1 Bar, Moshe (1 April 2000). "The Linux Scheduler". Linux Journal. Belltown Media, Inc.. Retrieved 14 April 2012. 
  125. "IEEE Standard for Information Technology – Portable Operating System Interface, POSIX.1b, Real-time extensions (IEEE Std 1003.1b-1993)". 
  126. Larabel, Michael (24 January 2014). "The Linux 3.14 Kernel Already Has Many Exciting Features". Phoronix. Retrieved 3 February 2014. 
  127. "Linux kernel 3.14, Section 1.1. Deadline scheduling class for better real-time scheduling". 30 March 2014. Retrieved 2 April 2014. 
  128. Love, Robert (2010). "4". Linux Kernel Development (3rd ed.). Addison Wesley. pp. 62-63. ISBN 9780672329463. 
  129. McKenney, Paul (10 August 2005). "A realtime preemption overview". Retrieved 5 February 2012. 
  130. "OSADL Project: Realtime Linux". OSADL. Retrieved 5 February 2012. 
  131. 131.0 131.1 131.2 Torvalds, Linus (January 1999). "The Linux Edge". Open Sources: Voices from the Open Source Revolution. O'Reilly. ISBN 1-56592-582-3. Retrieved 13 October 2013. 
  132. "Porting Linux to the DEC Alpha: The Kernel and Shell".,1. 
  133. "Linux on Alpha: A Strategic Choice".,0. 
  134. "Avalon Cluster | TOP500 Supercomputer Sites". 
  135. Wang, David (6 May 2010). "Android Now Running On iPhone 3G". TechHive. IDG. Retrieved 11 July 2010. 
  136. "The state of preempt-rt". Retrieved 14 June 2016. 
  137. Meyer, David (3 February 2010). "Linux developer explains Android kernel code removal". ZDNet. CBS Interactive. Retrieved 3 February 2010. 
  138. "Chapter 03: maemo Platform Overview". maemo Technology Overview. Nokia. 2008. Retrieved 9 April 2010. 
  139. Kroah-Hartman, Greg (2 February 2010). "Android and the Linux kernel community". Retrieved 3 February 2010. 
  140. 140.0 140.1 Roger Ye (2017). Android System Programming. Packt Publishing. p. 14. ISBN 9781787120389. 
  141. Bradford, John (8 March 2003). "Re: what's an OOPS". LKML (Mailing list). Retrieved 30 October 2010.
  142. "Linux kernel 4.0, Section 1.2. Live patching". 26 April 2015. Retrieved 27 April 2015. 
  143. Jonathan Corbet (25 February 2015). "A rough patch for live patching". Retrieved 27 April 2015. 
  144. "kernel/git/torvalds/linux.git: Pull live patching infrastructure from Jiri Kosina (Linux kernel source tree)". 11 February 2015. Retrieved 27 April 2015. 
  145. Mookhey, K. K.; Burghate, Nilesh (1 July 2005). Linux: Security, Audit and Control Features. USA: ISACA. p. 14. ISBN 1-893209-78-4. Retrieved 31 December 2010. 
  146. Hatch, Brian (15 July 2008). Hacking Exposed Linux: Linux Security Secrets and Solutions. McGraw-Hill Osborne Media. p. 524. ISBN 978-0-07-226257-5. Retrieved 31 December 2010. 
  147. Jaeger, Trent (7 October 2008). Operating System Security. Morgan and Claypool Publishers. p. 122. ISBN 978-1-59829-212-1. Retrieved 31 December 2010. 
  148. "Linux Security Module Usage — The Linux Kernel documentation". 
  149. "National Security Agency | Central Security Service > What We Do > Research > SE Linux > SE Linux FAQs". 
  150. "Seccomp BPF (SECure COMPuting with filters) — The Linux Kernel documentation". 
  151. Andrews, Jeremy (16 July 2008). "Security Bugs and Full Disclosure". KernelTrap. Archived from the original on 19 July 2008. Retrieved 31 December 2010. 
  152. Spengler, Brad (16 July 2008). "Linux's unofficial security-through-coverup policy". Full Disclosure (Mailing list). Retrieved 31 December 2010.
  153. "Kernel 1.0 Source Code Release". Retrieved 7 October 2008. 
  154. "Kernel 1.2 Source Code Release". Retrieved 27 October 2008. 
  155. Torvalds, Linus (9 June 1996). "Linux 2.0 really _is_ released." LKML (Mailing list). Retrieved 8 March 2015.
  156. Torvalds, Linus (20 January 1999). "2.2.0-final". LKML (Mailing list). Retrieved 8 March 2015.
  157. "The Wonderful World of Linux 2.2". 26 January 1999. Retrieved 27 October 2008. 
  158. "Linux/390 Observations and Notes". 
  159. Torvalds, Linus (4 January 2001). "And oh, btw." LKML (Mailing list). Retrieved 8 March 2015.
  160. "The Wonderful World of Linux 2.4". Retrieved 27 October 2008. 
  161. Torvalds, Linus (17 December 2003). "Linux 2.6.0". LKML (Mailing list). Retrieved 28 February 2015.
  162. "proc(5) - Linux manual page". 
  163. "Index of /pub/linux/kernel/v2.6". Retrieved 2 March 2014. 
  164. 164.0 164.1 Torvalds, Linus (21 July 2011). "Linux 3.0 release". Linux kernel mailing list. Retrieved 16 May 2013. 
  165. Torvalds, Linus (30 May 2011). "Linux 3.0-rc1". LKML (Mailing list). Archived from the original on 31 May 2011. Retrieved 1 July 2013.
  166. Vaughan-Nichols, Steven J. (13 December 2012). "Good-Bye 386: Linux to drop support for i386 chips with next major release". ZDNet. CBS Interactive. Retrieved 6 February 2013. 
  167. Fingas, Jon (15 December 2012). "Linux to drop i386 support in the 3.8 kernel, make us upgrade our Doom rig". Engadget. AOL. Retrieved 22 March 2015. 
  168. Vaughan-Nichols, Steven J. (11 December 2012). "Linux 3.7 arrives, ARM developers rejoice". ZDNet. CBS Interactive. Retrieved 6 February 2013. 
  169. Torvalds, Linus (2 September 2013). "Linux 3.11". LKML (Mailing list). Retrieved 3 September 2013.
  170. "Linux 3.11". 2 September 2013. Retrieved 21 January 2014. 
  171. "LKML: Linus Torvalds: Linux 5.1.15". 
  172. "Linux Kernel Features: Linus Torvalds: Linux 5.1.15". 13 April 2019. 
  173. Torvalds, Linus (12 April 2015). "Linux 4.0 released". LKML (Mailing list). Retrieved 12 April 2015.
  174. Foundation, The Linux (2017-10-25). "2017 State of Linux Kernel Development" (in en-US). 
  175. "How the development process works". Retrieved 4 February 2018. 
  176. Robert Love (2010). Linux Kernel Development: Linux Kernel Development. Pearson Education. p. 13. ISBN 9780768696790. 
  177. Gene Sally (2010). Pro Linux Embedded Systems. Apress. p. 252. ISBN 9781430272267. 
  178. "Code of Conflict". Retrieved 4 February 2018. [permanent dead link]
  179. Sharwood, Simon (6 October 2015). "Linux kernel dev who asked Linus Torvalds to stop verbal abuse quits over verbal abuse". 
  180. Edge, Jake (31 January 2018). "Too many lords, not enough stewards". Retrieved 4 February 2018. 
  181. Corbet, Jonathan (6 November 2017). "Bash the kernel maintainers". Retrieved 4 February 2018. 
  182. "Why I quit: kernel developer Con Kolivas". APC Magazine. ACP Magazines. 24 July 2007. Archived from the original on 2011-07-07. Retrieved 15 August 2011. 
  183. Corbet, Jonathan (25 July 2007). "Re: -mm merge plans for 2.6.23". Retrieved 10 February 2018. 
  184. Cox, Alan (28 July 2009). "Re: [PATCH kdesu broken"]. Retrieved 10 February 2018. 
  185. Rodrigues, Goldwyn (22 January 2011). "A tale of two SCSI targets". Retrieved 14 February 2018. 
  186. Steinmetz, Andreas (17 January 2013). "LIO - the broken iSCSI target implementation". Retrieved 14 February 2018. 
  187. Paul, Ryan (19 June 2012). "Linus Torvalds says "f–k you" to NVIDIA". Retrieved 14 February 2018. 
  188. John Gold (3 April 2014). "Linus Torvalds suspends key Linux developer: Kernel panic as Systemd dev pokes the bear". Retrieved 24 March 2019. 
  189. Poettering, Lennart (6 October 2014). "On the sickness of the Linux Kernel Community". Retrieved 10 February 2018. 
  190. Brodkin, Jon (6 March 2015). "VMware alleged to have violated Linux's open source license for years". Retrieved 14 February 2018. 
  191. McCarthy, Kieren (26 August 2016). "Having offended everyone else in the world, Linus Torvalds calls own lawyers a 'nasty festering disease'". Retrieved 14 February 2018. 
  192. Corbet, Jonathan (10 September 2007). "KS2007: Developer relations and development process". Retrieved 11 February 2018. 
  193. Brodkin, Jon (16 July 2013). "Linus Torvalds defends his right to shame Linux kernel developers". Retrieved 11 February 2018. 
  194. Corbet, Jonathan (9 March 2015). "The kernel's code of conflict". Retrieved 11 February 2018. 
  195. Corbet, Jonathan (2018-09-18). "Code, conflict, and conduct". 
  196. Cohen, Noam (2018-09-19). "After Years of Abusive E-mails, the Creator of Linux Steps Aside". The New Yorker. 
  197. Larabel, Michael. "Dropping Profanity In Kernel Code Comments: Linux Gets "Hugs"". Retrieved 15 June 2019. 
  198. "Linux Evolution". 26 March 2008. 
  199. "Perpetual Development: A Model of the Linux Kernel Life Cycle". 25 October 2011. 
  200. Kroah-Hartman, Greg (12 February 2008). "Re: Announce: Linux-next (Or Andrew's dream :-))". Linux Kernel Mailing List (Mailing list). Retrieved 30 January 2017.
  201. Wheeler, David A.. "Linux Kernel 2.6: It's Worth More!". 
  202. "Economic impact of FLOSS on innovation and competitiveness of the EU ICT sector". 
  203. "Estimating Total Development Cost Of a Linux Distribution". Archived from the original on 11 July 2010. 
  204. "The Billion Dollar Kernel". 24 February 2010. Retrieved 12 November 2010. 
  205. Wheeler, David. "The Linux Kernel: It's Worth More!". Retrieved 17 September 2012. 
  206. "Linux MAINTAINERS file".;a=blob;f=MAINTAINERS. 
  207. Torvalds, Linus (16 September 2018). "Linux 4.19-rc4 released, an apology, and a maintainership note". Retrieved 23 September 2018. 
  208. Alexandru Vaduva, Alex Gonzalez & Chris Simmonds (2016). Linux: Embedded Development. Packt Publishing. p. 663. ISBN 9781787124455. 
  209. "The Linux Kernel Archives". Retrieved 13 November 2019. 
  210. Robert Love (2010). Linux Kernel Development: Linux Kernel Development. Pearson Education. p. 11. ISBN 9780768696790. 
  211. Robert Love (2010). Linux Kernel Development: Linux Kernel Development. Pearson Education. p. 12. ISBN 9780768696790. 
  212. 212.0 212.1 Yamagata, Hiroo (3 August 1997). "The Pragmatist of Free Software". HotWired. Archived from the original on 10 February 2007. Retrieved 21 February 2007. 
  213. "GPL-v2". 
  214. Corbet, Jonathan (31 January 2006). "GPLv3 and the kernel". Retrieved 21 February 2007. 
  215. Torvalds, Linus (8 September 2000). "Linux-2.4.0-test8". LKML (Mailing list). Retrieved 21 February 2007.
  216. "" (in en). 
  217. Cox, Alan (20 January 2006). "Re: GPL V3 and Linux". LKML (Mailing list). Retrieved 21 February 2007.
  218. Shankland, Stephen (25 September 2006). "Top Linux programmers pan GPL 3". CNET. Retrieved 21 February 2007. 
  219. 219.0 219.1 "Kernel developers' position on GPLv3: The Dangers and Problems with GPLv3". 15 September 2006. Retrieved 11 March 2015. 
  220. Petreley, Nicholas (27 September 2006). "A fight against evil or a fight for attention?". Retrieved 11 March 2015. 
  221. "Linus Torvalds says GPL v3 violates everything that GPLv2 stood for". Debconf 2014. 2014. Retrieved 21 March 2018. 
  222. Clark, Rob; Semwal, Sumit (1 November 2012). "DMA Buffer Sharing Framework: An Introduction". Embedded Linux Conference. Retrieved 2 August 2014. 
  223. Cox, Alan (10 October 2012). "[PATCH] dma-buf: Use EXPORT_SYMBOL". Direct Rendering Infrastructure (Mailing list). Retrieved 3 September 2013.
  224. Torvalds, Linus (10 December 2003). "RE: Linux GPL and binary module exception clause?". LKML (Mailing list). Retrieved 31 December 2010.
  225. Torvalds, Linus (3 December 2003). "Re: Linux GPL and binary module exception clause?". LKML (Mailing list). Retrieved 12 November 2010.
  226. "Linux Firmware API — The Linux Kernel documentation". 
  227. "Tainted kernels — The Linux Kernel documentation". 
  228. "Built-in firmware — The Linux Kernel documentation". 
  229. "Linux TM registration in the US". 
  230. "Linux TM registration in the EU". 
  231. Hughes, Phil (1 August 1997). "Linux Trademark Dispute". Linux Journal. Belltown Media, Inc.. Retrieved 8 December 2010. 
  232. Hughes, Phil (1 March 1997). "Action Taken on Linux Trademark". Linux Journal. Belltown Media, Inc.. Retrieved 8 December 2010. 
  233. Gisselberg, Tonya (2010). "The Trademark History of Linux, the Operating System". Gisselberg Law Firm, Inc.. Retrieved 8 December 2010. 

Further reading

External links