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+Linux kernel development in the early 1990's was a pretty loose affair,
+with relatively small numbers of users and developers involved. With a
+user base in the millions and with some 2,000 developers involved over the
+course of one year, the kernel has since had to evolve a number of
+processes to keep development happening smoothly. A solid understanding of
+how the process works is required in order to be an effective part of it.
+The kernel developers use a loosely time-based release process, with a new
+major kernel release happening every two or three months. The recent
+release history looks like this:
+ 2.6.38 March 14, 2011
+ 2.6.37 January 4, 2011
+ 2.6.36 October 20, 2010
+ 2.6.35 August 1, 2010
+ 2.6.34 May 15, 2010
+ 2.6.33 February 24, 2010
+Every 2.6.x release is a major kernel release with new features, internal
+API changes, and more. A typical 2.6 release can contain nearly 10,000
+changesets with changes to several hundred thousand lines of code. 2.6 is
+thus the leading edge of Linux kernel development; the kernel uses a
+rolling development model which is continually integrating major changes.
+A relatively straightforward discipline is followed with regard to the
+merging of patches for each release. At the beginning of each development
+cycle, the "merge window" is said to be open. At that time, code which is
+deemed to be sufficiently stable (and which is accepted by the development
+community) is merged into the mainline kernel. The bulk of changes for a
+new development cycle (and all of the major changes) will be merged during
+this time, at a rate approaching 1,000 changes ("patches," or "changesets")
+per day.
+(As an aside, it is worth noting that the changes integrated during the
+merge window do not come out of thin air; they have been collected, tested,
+and staged ahead of time. How that process works will be described in
+detail later on).
+The merge window lasts for approximately two weeks. At the end of this
+time, Linus Torvalds will declare that the window is closed and release the
+first of the "rc" kernels. For the kernel which is destined to be 2.6.40,
+for example, the release which happens at the end of the merge window will
+be called 2.6.40-rc1. The -rc1 release is the signal that the time to
+merge new features has passed, and that the time to stabilize the next
+kernel has begun.
+Over the next six to ten weeks, only patches which fix problems should be
+submitted to the mainline. On occasion a more significant change will be
+allowed, but such occasions are rare; developers who try to merge new
+features outside of the merge window tend to get an unfriendly reception.
+As a general rule, if you miss the merge window for a given feature, the
+best thing to do is to wait for the next development cycle. (An occasional
+exception is made for drivers for previously-unsupported hardware; if they
+touch no in-tree code, they cannot cause regressions and should be safe to
+add at any time).
+As fixes make their way into the mainline, the patch rate will slow over
+time. Linus releases new -rc kernels about once a week; a normal series
+will get up to somewhere between -rc6 and -rc9 before the kernel is
+considered to be sufficiently stable and the final 2.6.x release is made.
+At that point the whole process starts over again.
+As an example, here is how the 2.6.38 development cycle went (all dates in
+ January 4 2.6.37 stable release
+ January 18 2.6.38-rc1, merge window closes
+ January 21 2.6.38-rc2
+ February 1 2.6.38-rc3
+ February 7 2.6.38-rc4
+ February 15 2.6.38-rc5
+ February 21 2.6.38-rc6
+ March 1 2.6.38-rc7
+ March 7 2.6.38-rc8
+ March 14 2.6.38 stable release
+How do the developers decide when to close the development cycle and create
+the stable release? The most significant metric used is the list of
+regressions from previous releases. No bugs are welcome, but those which
+break systems which worked in the past are considered to be especially
+serious. For this reason, patches which cause regressions are looked upon
+unfavorably and are quite likely to be reverted during the stabilization
+The developers' goal is to fix all known regressions before the stable
+release is made. In the real world, this kind of perfection is hard to
+achieve; there are just too many variables in a project of this size.
+There comes a point where delaying the final release just makes the problem
+worse; the pile of changes waiting for the next merge window will grow
+larger, creating even more regressions the next time around. So most 2.6.x
+kernels go out with a handful of known regressions though, hopefully, none
+of them are serious.
+Once a stable release is made, its ongoing maintenance is passed off to the
+"stable team," currently consisting of Greg Kroah-Hartman. The stable team
+will release occasional updates to the stable release using the 2.6.x.y
+numbering scheme. To be considered for an update release, a patch must (1)
+fix a significant bug, and (2) already be merged into the mainline for the
+next development kernel. Kernels will typically receive stable updates for
+a little more than one development cycle past their initial release. So,
+for example, the 2.6.36 kernel's history looked like:
+ October 10 2.6.36 stable release
+ November 22
+ December 9
+ January 7
+ February 17
+ was the final stable update for the 2.6.36 release.
+Some kernels are designated "long term" kernels; they will receive support
+for a longer period. As of this writing, the current long term kernels
+and their maintainers are:
+ 2.6.27 Willy Tarreau (Deep-frozen stable kernel)
+ 2.6.32 Greg Kroah-Hartman
+ 2.6.35 Andi Kleen (Embedded flag kernel)
+The selection of a kernel for long-term support is purely a matter of a
+maintainer having the need and the time to maintain that release. There
+are no known plans for long-term support for any specific upcoming
+Patches do not go directly from the developer's keyboard into the mainline
+kernel. There is, instead, a somewhat involved (if somewhat informal)
+process designed to ensure that each patch is reviewed for quality and that
+each patch implements a change which is desirable to have in the mainline.
+This process can happen quickly for minor fixes, or, in the case of large
+and controversial changes, go on for years. Much developer frustration
+comes from a lack of understanding of this process or from attempts to
+circumvent it.
+In the hopes of reducing that frustration, this document will describe how
+a patch gets into the kernel. What follows below is an introduction which
+describes the process in a somewhat idealized way. A much more detailed
+treatment will come in later sections.
+The stages that a patch goes through are, generally:
+ - Design. This is where the real requirements for the patch - and the way
+ those requirements will be met - are laid out. Design work is often
+ done without involving the community, but it is better to do this work
+ in the open if at all possible; it can save a lot of time redesigning
+ things later.
+ - Early review. Patches are posted to the relevant mailing list, and
+ developers on that list reply with any comments they may have. This
+ process should turn up any major problems with a patch if all goes
+ well.
+ - Wider review. When the patch is getting close to ready for mainline
+ inclusion, it should be accepted by a relevant subsystem maintainer -
+ though this acceptance is not a guarantee that the patch will make it
+ all the way to the mainline. The patch will show up in the maintainer's
+ subsystem tree and into the -next trees (described below). When the
+ process works, this step leads to more extensive review of the patch and
+ the discovery of any problems resulting from the integration of this
+ patch with work being done by others.
+- Please note that most maintainers also have day jobs, so merging
+ your patch may not be their highest priority. If your patch is
+ getting feedback about changes that are needed, you should either
+ make those changes or justify why they should not be made. If your
+ patch has no review complaints but is not being merged by its
+ appropriate subsystem or driver maintainer, you should be persistent
+ in updating the patch to the current kernel so that it applies cleanly
+ and keep sending it for review and merging.
+ - Merging into the mainline. Eventually, a successful patch will be
+ merged into the mainline repository managed by Linus Torvalds. More
+ comments and/or problems may surface at this time; it is important that
+ the developer be responsive to these and fix any issues which arise.
+ - Stable release. The number of users potentially affected by the patch
+ is now large, so, once again, new problems may arise.
+ - Long-term maintenance. While it is certainly possible for a developer
+ to forget about code after merging it, that sort of behavior tends to
+ leave a poor impression in the development community. Merging code
+ eliminates some of the maintenance burden, in that others will fix
+ problems caused by API changes. But the original developer should
+ continue to take responsibility for the code if it is to remain useful
+ in the longer term.
+One of the largest mistakes made by kernel developers (or their employers)
+is to try to cut the process down to a single "merging into the mainline"
+step. This approach invariably leads to frustration for everybody
+There is exactly one person who can merge patches into the mainline kernel
+repository: Linus Torvalds. But, of the over 9,500 patches which went
+into the 2.6.38 kernel, only 112 (around 1.3%) were directly chosen by Linus
+himself. The kernel project has long since grown to a size where no single
+developer could possibly inspect and select every patch unassisted. The
+way the kernel developers have addressed this growth is through the use of
+a lieutenant system built around a chain of trust.
+The kernel code base is logically broken down into a set of subsystems:
+networking, specific architecture support, memory management, video
+devices, etc. Most subsystems have a designated maintainer, a developer
+who has overall responsibility for the code within that subsystem. These
+subsystem maintainers are the gatekeepers (in a loose way) for the portion
+of the kernel they manage; they are the ones who will (usually) accept a
+patch for inclusion into the mainline kernel.
+Subsystem maintainers each manage their own version of the kernel source
+tree, usually (but certainly not always) using the git source management
+tool. Tools like git (and related tools like quilt or mercurial) allow
+maintainers to track a list of patches, including authorship information
+and other metadata. At any given time, the maintainer can identify which
+patches in his or her repository are not found in the mainline.
+When the merge window opens, top-level maintainers will ask Linus to "pull"
+the patches they have selected for merging from their repositories. If
+Linus agrees, the stream of patches will flow up into his repository,
+becoming part of the mainline kernel. The amount of attention that Linus
+pays to specific patches received in a pull operation varies. It is clear
+that, sometimes, he looks quite closely. But, as a general rule, Linus
+trusts the subsystem maintainers to not send bad patches upstream.
+Subsystem maintainers, in turn, can pull patches from other maintainers.
+For example, the networking tree is built from patches which accumulated
+first in trees dedicated to network device drivers, wireless networking,
+etc. This chain of repositories can be arbitrarily long, though it rarely
+exceeds two or three links. Since each maintainer in the chain trusts
+those managing lower-level trees, this process is known as the "chain of
+Clearly, in a system like this, getting patches into the kernel depends on
+finding the right maintainer. Sending patches directly to Linus is not
+normally the right way to go.
+The chain of subsystem trees guides the flow of patches into the kernel,
+but it also raises an interesting question: what if somebody wants to look
+at all of the patches which are being prepared for the next merge window?
+Developers will be interested in what other changes are pending to see
+whether there are any conflicts to worry about; a patch which changes a
+core kernel function prototype, for example, will conflict with any other
+patches which use the older form of that function. Reviewers and testers
+want access to the changes in their integrated form before all of those
+changes land in the mainline kernel. One could pull changes from all of
+the interesting subsystem trees, but that would be a big and error-prone
+The answer comes in the form of -next trees, where subsystem trees are
+collected for testing and review. The older of these trees, maintained by
+Andrew Morton, is called "-mm" (for memory management, which is how it got
+started). The -mm tree integrates patches from a long list of subsystem
+trees; it also has some patches aimed at helping with debugging.
+Beyond that, -mm contains a significant collection of patches which have
+been selected by Andrew directly. These patches may have been posted on a
+mailing list, or they may apply to a part of the kernel for which there is
+no designated subsystem tree. As a result, -mm operates as a sort of
+subsystem tree of last resort; if there is no other obvious path for a
+patch into the mainline, it is likely to end up in -mm. Miscellaneous
+patches which accumulate in -mm will eventually either be forwarded on to
+an appropriate subsystem tree or be sent directly to Linus. In a typical
+development cycle, approximately 5-10% of the patches going into the
+mainline get there via -mm.
+The current -mm patch is available in the "mmotm" (-mm of the moment)
+directory at:
+ http://userweb.kernel.org/~akpm/mmotm/
+Use of the MMOTM tree is likely to be a frustrating experience, though;
+there is a definite chance that it will not even compile.
+The primary tree for next-cycle patch merging is linux-next, maintained by
+Stephen Rothwell. The linux-next tree is, by design, a snapshot of what
+the mainline is expected to look like after the next merge window closes.
+Linux-next trees are announced on the linux-kernel and linux-next mailing
+lists when they are assembled; they can be downloaded from:
+ http://www.kernel.org/pub/linux/kernel/people/sfr/linux-next/
+Some information about linux-next has been gathered at:
+ http://linux.f-seidel.de/linux-next/pmwiki/
+Linux-next has become an integral part of the kernel development process;
+all patches merged during a given merge window should really have found
+their way into linux-next some time before the merge window opens.
+The kernel source tree contains the drivers/staging/ directory, where
+many sub-directories for drivers or filesystems that are on their way to
+being added to the kernel tree live. They remain in drivers/staging while
+they still need more work; once complete, they can be moved into the
+kernel proper. This is a way to keep track of drivers that aren't
+up to Linux kernel coding or quality standards, but people may want to use
+them and track development.
+Greg Kroah-Hartman currently maintains the staging tree. Drivers that
+still need work are sent to him, with each driver having its own
+subdirectory in drivers/staging/. Along with the driver source files, a
+TODO file should be present in the directory as well. The TODO file lists
+the pending work that the driver needs for acceptance into the kernel
+proper, as well as a list of people that should be Cc'd for any patches to
+the driver. Current rules require that drivers contributed to staging
+must, at a minimum, compile properly.
+Staging can be a relatively easy way to get new drivers into the mainline
+where, with luck, they will come to the attention of other developers and
+improve quickly. Entry into staging is not the end of the story, though;
+code in staging which is not seeing regular progress will eventually be
+removed. Distributors also tend to be relatively reluctant to enable
+staging drivers. So staging is, at best, a stop on the way toward becoming
+a proper mainline driver.
+2.5: TOOLS
+As can be seen from the above text, the kernel development process depends
+heavily on the ability to herd collections of patches in various
+directions. The whole thing would not work anywhere near as well as it
+does without suitably powerful tools. Tutorials on how to use these tools
+are well beyond the scope of this document, but there is space for a few
+By far the dominant source code management system used by the kernel
+community is git. Git is one of a number of distributed version control
+systems being developed in the free software community. It is well tuned
+for kernel development, in that it performs quite well when dealing with
+large repositories and large numbers of patches. It also has a reputation
+for being difficult to learn and use, though it has gotten better over
+time. Some sort of familiarity with git is almost a requirement for kernel
+developers; even if they do not use it for their own work, they'll need git
+to keep up with what other developers (and the mainline) are doing.
+Git is now packaged by almost all Linux distributions. There is a home
+page at:
+ http://git-scm.com/
+That page has pointers to documentation and tutorials.
+Among the kernel developers who do not use git, the most popular choice is
+almost certainly Mercurial:
+ http://www.selenic.com/mercurial/
+Mercurial shares many features with git, but it provides an interface which
+many find easier to use.
+The other tool worth knowing about is Quilt:
+ http://savannah.nongnu.org/projects/quilt/
+Quilt is a patch management system, rather than a source code management
+system. It does not track history over time; it is, instead, oriented
+toward tracking a specific set of changes against an evolving code base.
+Some major subsystem maintainers use quilt to manage patches intended to go
+upstream. For the management of certain kinds of trees (-mm, for example),
+quilt is the best tool for the job.
+A great deal of Linux kernel development work is done by way of mailing
+lists. It is hard to be a fully-functioning member of the community
+without joining at least one list somewhere. But Linux mailing lists also
+represent a potential hazard to developers, who risk getting buried under a
+load of electronic mail, running afoul of the conventions used on the Linux
+lists, or both.
+Most kernel mailing lists are run on vger.kernel.org; the master list can
+be found at:
+ http://vger.kernel.org/vger-lists.html
+There are lists hosted elsewhere, though; a number of them are at
+The core mailing list for kernel development is, of course, linux-kernel.
+This list is an intimidating place to be; volume can reach 500 messages per
+day, the amount of noise is high, the conversation can be severely
+technical, and participants are not always concerned with showing a high
+degree of politeness. But there is no other place where the kernel
+development community comes together as a whole; developers who avoid this
+list will miss important information.
+There are a few hints which can help with linux-kernel survival:
+- Have the list delivered to a separate folder, rather than your main
+ mailbox. One must be able to ignore the stream for sustained periods of
+ time.
+- Do not try to follow every conversation - nobody else does. It is
+ important to filter on both the topic of interest (though note that
+ long-running conversations can drift away from the original subject
+ without changing the email subject line) and the people who are
+ participating.
+- Do not feed the trolls. If somebody is trying to stir up an angry
+ response, ignore them.
+- When responding to linux-kernel email (or that on other lists) preserve
+ the Cc: header for all involved. In the absence of a strong reason (such
+ as an explicit request), you should never remove recipients. Always make
+ sure that the person you are responding to is in the Cc: list. This
+ convention also makes it unnecessary to explicitly ask to be copied on
+ replies to your postings.
+- Search the list archives (and the net as a whole) before asking
+ questions. Some developers can get impatient with people who clearly
+ have not done their homework.
+- Avoid top-posting (the practice of putting your answer above the quoted
+ text you are responding to). It makes your response harder to read and
+ makes a poor impression.
+- Ask on the correct mailing list. Linux-kernel may be the general meeting
+ point, but it is not the best place to find developers from all
+ subsystems.
+The last point - finding the correct mailing list - is a common place for
+beginning developers to go wrong. Somebody who asks a networking-related
+question on linux-kernel will almost certainly receive a polite suggestion
+to ask on the netdev list instead, as that is the list frequented by most
+networking developers. Other lists exist for the SCSI, video4linux, IDE,
+filesystem, etc. subsystems. The best place to look for mailing lists is
+in the MAINTAINERS file packaged with the kernel source.
+Questions about how to get started with the kernel development process are
+common - from both individuals and companies. Equally common are missteps
+which make the beginning of the relationship harder than it has to be.
+Companies often look to hire well-known developers to get a development
+group started. This can, in fact, be an effective technique. But it also
+tends to be expensive and does not do much to grow the pool of experienced
+kernel developers. It is possible to bring in-house developers up to speed
+on Linux kernel development, given the investment of a bit of time. Taking
+this time can endow an employer with a group of developers who understand
+the kernel and the company both, and who can help to train others as well.
+Over the medium term, this is often the more profitable approach.
+Individual developers are often, understandably, at a loss for a place to
+start. Beginning with a large project can be intimidating; one often wants
+to test the waters with something smaller first. This is the point where
+some developers jump into the creation of patches fixing spelling errors or
+minor coding style issues. Unfortunately, such patches create a level of
+noise which is distracting for the development community as a whole, so,
+increasingly, they are looked down upon. New developers wishing to
+introduce themselves to the community will not get the sort of reception
+they wish for by these means.
+Andrew Morton gives this advice for aspiring kernel developers
+ The #1 project for all kernel beginners should surely be "make sure
+ that the kernel runs perfectly at all times on all machines which
+ you can lay your hands on". Usually the way to do this is to work
+ with others on getting things fixed up (this can require
+ persistence!) but that's fine - it's a part of kernel development.
+In the absence of obvious problems to fix, developers are advised to look
+at the current lists of regressions and open bugs in general. There is
+never any shortage of issues in need of fixing; by addressing these issues,
+developers will gain experience with the process while, at the same time,
+building respect with the rest of the development community.