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Jonathan Corbet75b02142008-09-30 15:15:56 -060012: HOW THE DEVELOPMENT PROCESS WORKS
2
3Linux kernel development in the early 1990's was a pretty loose affair,
4with relatively small numbers of users and developers involved. With a
5user base in the millions and with some 2,000 developers involved over the
6course of one year, the kernel has since had to evolve a number of
7processes to keep development happening smoothly. A solid understanding of
8how the process works is required in order to be an effective part of it.
9
10
112.1: THE BIG PICTURE
12
13The kernel developers use a loosely time-based release process, with a new
14major kernel release happening every two or three months. The recent
15release history looks like this:
16
Jonathan Corbet5c050fb2011-03-25 12:17:53 -060017 2.6.38 March 14, 2011
18 2.6.37 January 4, 2011
19 2.6.36 October 20, 2010
20 2.6.35 August 1, 2010
21 2.6.34 May 15, 2010
22 2.6.33 February 24, 2010
Jonathan Corbet75b02142008-09-30 15:15:56 -060023
24Every 2.6.x release is a major kernel release with new features, internal
Jonathan Corbet5c050fb2011-03-25 12:17:53 -060025API changes, and more. A typical 2.6 release can contain nearly 10,000
Jonathan Corbet75b02142008-09-30 15:15:56 -060026changesets with changes to several hundred thousand lines of code. 2.6 is
27thus the leading edge of Linux kernel development; the kernel uses a
28rolling development model which is continually integrating major changes.
29
30A relatively straightforward discipline is followed with regard to the
31merging of patches for each release. At the beginning of each development
32cycle, the "merge window" is said to be open. At that time, code which is
33deemed to be sufficiently stable (and which is accepted by the development
34community) is merged into the mainline kernel. The bulk of changes for a
35new development cycle (and all of the major changes) will be merged during
36this time, at a rate approaching 1,000 changes ("patches," or "changesets")
37per day.
38
39(As an aside, it is worth noting that the changes integrated during the
40merge window do not come out of thin air; they have been collected, tested,
41and staged ahead of time. How that process works will be described in
42detail later on).
43
Jonathan Corbet5c050fb2011-03-25 12:17:53 -060044The merge window lasts for approximately two weeks. At the end of this
45time, Linus Torvalds will declare that the window is closed and release the
46first of the "rc" kernels. For the kernel which is destined to be 2.6.40,
47for example, the release which happens at the end of the merge window will
48be called 2.6.40-rc1. The -rc1 release is the signal that the time to
49merge new features has passed, and that the time to stabilize the next
50kernel has begun.
Jonathan Corbet75b02142008-09-30 15:15:56 -060051
52Over the next six to ten weeks, only patches which fix problems should be
53submitted to the mainline. On occasion a more significant change will be
54allowed, but such occasions are rare; developers who try to merge new
55features outside of the merge window tend to get an unfriendly reception.
56As a general rule, if you miss the merge window for a given feature, the
57best thing to do is to wait for the next development cycle. (An occasional
58exception is made for drivers for previously-unsupported hardware; if they
59touch no in-tree code, they cannot cause regressions and should be safe to
60add at any time).
61
62As fixes make their way into the mainline, the patch rate will slow over
63time. Linus releases new -rc kernels about once a week; a normal series
64will get up to somewhere between -rc6 and -rc9 before the kernel is
65considered to be sufficiently stable and the final 2.6.x release is made.
66At that point the whole process starts over again.
67
Jonathan Corbet5c050fb2011-03-25 12:17:53 -060068As an example, here is how the 2.6.38 development cycle went (all dates in
692011):
Jonathan Corbet75b02142008-09-30 15:15:56 -060070
Jonathan Corbet5c050fb2011-03-25 12:17:53 -060071 January 4 2.6.37 stable release
72 January 18 2.6.38-rc1, merge window closes
73 January 21 2.6.38-rc2
74 February 1 2.6.38-rc3
75 February 7 2.6.38-rc4
76 February 15 2.6.38-rc5
77 February 21 2.6.38-rc6
78 March 1 2.6.38-rc7
79 March 7 2.6.38-rc8
80 March 14 2.6.38 stable release
Jonathan Corbet75b02142008-09-30 15:15:56 -060081
82How do the developers decide when to close the development cycle and create
83the stable release? The most significant metric used is the list of
84regressions from previous releases. No bugs are welcome, but those which
85break systems which worked in the past are considered to be especially
86serious. For this reason, patches which cause regressions are looked upon
87unfavorably and are quite likely to be reverted during the stabilization
Jonathan Corbet5c050fb2011-03-25 12:17:53 -060088period.
Jonathan Corbet75b02142008-09-30 15:15:56 -060089
90The developers' goal is to fix all known regressions before the stable
91release is made. In the real world, this kind of perfection is hard to
92achieve; there are just too many variables in a project of this size.
93There comes a point where delaying the final release just makes the problem
94worse; the pile of changes waiting for the next merge window will grow
95larger, creating even more regressions the next time around. So most 2.6.x
96kernels go out with a handful of known regressions though, hopefully, none
97of them are serious.
98
99Once a stable release is made, its ongoing maintenance is passed off to the
Jonathan Corbet5c050fb2011-03-25 12:17:53 -0600100"stable team," currently consisting of Greg Kroah-Hartman. The stable team
101will release occasional updates to the stable release using the 2.6.x.y
102numbering scheme. To be considered for an update release, a patch must (1)
103fix a significant bug, and (2) already be merged into the mainline for the
104next development kernel. Kernels will typically receive stable updates for
105a little more than one development cycle past their initial release. So,
106for example, the 2.6.36 kernel's history looked like:
Jonathan Corbet75b02142008-09-30 15:15:56 -0600107
Jonathan Corbet5c050fb2011-03-25 12:17:53 -0600108 October 10 2.6.36 stable release
109 November 22 2.6.36.1
110 December 9 2.6.36.2
111 January 7 2.6.36.3
112 February 17 2.6.36.4
Jonathan Corbet75b02142008-09-30 15:15:56 -0600113
Jonathan Corbet5c050fb2011-03-25 12:17:53 -06001142.6.36.4 was the final stable update for the 2.6.36 release.
115
116Some kernels are designated "long term" kernels; they will receive support
117for a longer period. As of this writing, the current long term kernels
118and their maintainers are:
119
120 2.6.27 Willy Tarreau (Deep-frozen stable kernel)
121 2.6.32 Greg Kroah-Hartman
122 2.6.35 Andi Kleen (Embedded flag kernel)
123
124The selection of a kernel for long-term support is purely a matter of a
125maintainer having the need and the time to maintain that release. There
126are no known plans for long-term support for any specific upcoming
127release.
Jonathan Corbet75b02142008-09-30 15:15:56 -0600128
129
1302.2: THE LIFECYCLE OF A PATCH
131
132Patches do not go directly from the developer's keyboard into the mainline
133kernel. There is, instead, a somewhat involved (if somewhat informal)
134process designed to ensure that each patch is reviewed for quality and that
135each patch implements a change which is desirable to have in the mainline.
136This process can happen quickly for minor fixes, or, in the case of large
137and controversial changes, go on for years. Much developer frustration
138comes from a lack of understanding of this process or from attempts to
Jonathan Corbet5c050fb2011-03-25 12:17:53 -0600139circumvent it.
Jonathan Corbet75b02142008-09-30 15:15:56 -0600140
141In the hopes of reducing that frustration, this document will describe how
142a patch gets into the kernel. What follows below is an introduction which
143describes the process in a somewhat idealized way. A much more detailed
144treatment will come in later sections.
145
146The stages that a patch goes through are, generally:
147
148 - Design. This is where the real requirements for the patch - and the way
149 those requirements will be met - are laid out. Design work is often
150 done without involving the community, but it is better to do this work
151 in the open if at all possible; it can save a lot of time redesigning
152 things later.
153
154 - Early review. Patches are posted to the relevant mailing list, and
155 developers on that list reply with any comments they may have. This
156 process should turn up any major problems with a patch if all goes
157 well.
158
159 - Wider review. When the patch is getting close to ready for mainline
Randy Dunlapef0eba42010-05-23 17:02:30 -0700160 inclusion, it should be accepted by a relevant subsystem maintainer -
Jonathan Corbet75b02142008-09-30 15:15:56 -0600161 though this acceptance is not a guarantee that the patch will make it
162 all the way to the mainline. The patch will show up in the maintainer's
Andres Salomone4fabad2010-11-18 12:27:35 -0800163 subsystem tree and into the -next trees (described below). When the
Jonathan Corbet75b02142008-09-30 15:15:56 -0600164 process works, this step leads to more extensive review of the patch and
165 the discovery of any problems resulting from the integration of this
166 patch with work being done by others.
167
Randy Dunlapef0eba42010-05-23 17:02:30 -0700168- Please note that most maintainers also have day jobs, so merging
169 your patch may not be their highest priority. If your patch is
170 getting feedback about changes that are needed, you should either
171 make those changes or justify why they should not be made. If your
172 patch has no review complaints but is not being merged by its
173 appropriate subsystem or driver maintainer, you should be persistent
174 in updating the patch to the current kernel so that it applies cleanly
175 and keep sending it for review and merging.
176
Jonathan Corbet75b02142008-09-30 15:15:56 -0600177 - Merging into the mainline. Eventually, a successful patch will be
178 merged into the mainline repository managed by Linus Torvalds. More
179 comments and/or problems may surface at this time; it is important that
180 the developer be responsive to these and fix any issues which arise.
181
182 - Stable release. The number of users potentially affected by the patch
183 is now large, so, once again, new problems may arise.
184
185 - Long-term maintenance. While it is certainly possible for a developer
186 to forget about code after merging it, that sort of behavior tends to
187 leave a poor impression in the development community. Merging code
188 eliminates some of the maintenance burden, in that others will fix
189 problems caused by API changes. But the original developer should
190 continue to take responsibility for the code if it is to remain useful
191 in the longer term.
192
193One of the largest mistakes made by kernel developers (or their employers)
194is to try to cut the process down to a single "merging into the mainline"
195step. This approach invariably leads to frustration for everybody
196involved.
197
198
1992.3: HOW PATCHES GET INTO THE KERNEL
200
201There is exactly one person who can merge patches into the mainline kernel
Jonathan Corbet5c050fb2011-03-25 12:17:53 -0600202repository: Linus Torvalds. But, of the over 9,500 patches which went
203into the 2.6.38 kernel, only 112 (around 1.3%) were directly chosen by Linus
Jonathan Corbet75b02142008-09-30 15:15:56 -0600204himself. The kernel project has long since grown to a size where no single
205developer could possibly inspect and select every patch unassisted. The
206way the kernel developers have addressed this growth is through the use of
207a lieutenant system built around a chain of trust.
208
209The kernel code base is logically broken down into a set of subsystems:
210networking, specific architecture support, memory management, video
211devices, etc. Most subsystems have a designated maintainer, a developer
212who has overall responsibility for the code within that subsystem. These
213subsystem maintainers are the gatekeepers (in a loose way) for the portion
214of the kernel they manage; they are the ones who will (usually) accept a
215patch for inclusion into the mainline kernel.
216
217Subsystem maintainers each manage their own version of the kernel source
218tree, usually (but certainly not always) using the git source management
219tool. Tools like git (and related tools like quilt or mercurial) allow
220maintainers to track a list of patches, including authorship information
221and other metadata. At any given time, the maintainer can identify which
222patches in his or her repository are not found in the mainline.
223
224When the merge window opens, top-level maintainers will ask Linus to "pull"
225the patches they have selected for merging from their repositories. If
226Linus agrees, the stream of patches will flow up into his repository,
227becoming part of the mainline kernel. The amount of attention that Linus
228pays to specific patches received in a pull operation varies. It is clear
229that, sometimes, he looks quite closely. But, as a general rule, Linus
230trusts the subsystem maintainers to not send bad patches upstream.
231
232Subsystem maintainers, in turn, can pull patches from other maintainers.
233For example, the networking tree is built from patches which accumulated
234first in trees dedicated to network device drivers, wireless networking,
235etc. This chain of repositories can be arbitrarily long, though it rarely
236exceeds two or three links. Since each maintainer in the chain trusts
237those managing lower-level trees, this process is known as the "chain of
Jonathan Corbet5c050fb2011-03-25 12:17:53 -0600238trust."
Jonathan Corbet75b02142008-09-30 15:15:56 -0600239
240Clearly, in a system like this, getting patches into the kernel depends on
241finding the right maintainer. Sending patches directly to Linus is not
242normally the right way to go.
243
244
Andres Salomone4fabad2010-11-18 12:27:35 -08002452.4: NEXT TREES
Jonathan Corbet75b02142008-09-30 15:15:56 -0600246
247The chain of subsystem trees guides the flow of patches into the kernel,
248but it also raises an interesting question: what if somebody wants to look
249at all of the patches which are being prepared for the next merge window?
250Developers will be interested in what other changes are pending to see
251whether there are any conflicts to worry about; a patch which changes a
252core kernel function prototype, for example, will conflict with any other
253patches which use the older form of that function. Reviewers and testers
254want access to the changes in their integrated form before all of those
255changes land in the mainline kernel. One could pull changes from all of
256the interesting subsystem trees, but that would be a big and error-prone
257job.
258
Andres Salomone4fabad2010-11-18 12:27:35 -0800259The answer comes in the form of -next trees, where subsystem trees are
Jonathan Corbet75b02142008-09-30 15:15:56 -0600260collected for testing and review. The older of these trees, maintained by
261Andrew Morton, is called "-mm" (for memory management, which is how it got
262started). The -mm tree integrates patches from a long list of subsystem
Jonathan Corbet5c050fb2011-03-25 12:17:53 -0600263trees; it also has some patches aimed at helping with debugging.
Jonathan Corbet75b02142008-09-30 15:15:56 -0600264
265Beyond that, -mm contains a significant collection of patches which have
266been selected by Andrew directly. These patches may have been posted on a
267mailing list, or they may apply to a part of the kernel for which there is
268no designated subsystem tree. As a result, -mm operates as a sort of
269subsystem tree of last resort; if there is no other obvious path for a
270patch into the mainline, it is likely to end up in -mm. Miscellaneous
271patches which accumulate in -mm will eventually either be forwarded on to
272an appropriate subsystem tree or be sent directly to Linus. In a typical
Jonathan Corbet5c050fb2011-03-25 12:17:53 -0600273development cycle, approximately 5-10% of the patches going into the
274mainline get there via -mm.
Jonathan Corbet75b02142008-09-30 15:15:56 -0600275
Randy Dunlape6a591e2010-05-23 17:02:30 -0700276The current -mm patch is available in the "mmotm" (-mm of the moment)
277directory at:
Jonathan Corbet75b02142008-09-30 15:15:56 -0600278
279 http://userweb.kernel.org/~akpm/mmotm/
280
281Use of the MMOTM tree is likely to be a frustrating experience, though;
282there is a definite chance that it will not even compile.
283
Jonathan Corbet5c050fb2011-03-25 12:17:53 -0600284The primary tree for next-cycle patch merging is linux-next, maintained by
Jonathan Corbet75b02142008-09-30 15:15:56 -0600285Stephen Rothwell. The linux-next tree is, by design, a snapshot of what
286the mainline is expected to look like after the next merge window closes.
287Linux-next trees are announced on the linux-kernel and linux-next mailing
288lists when they are assembled; they can be downloaded from:
289
290 http://www.kernel.org/pub/linux/kernel/people/sfr/linux-next/
291
292Some information about linux-next has been gathered at:
293
294 http://linux.f-seidel.de/linux-next/pmwiki/
295
Jonathan Corbet5c050fb2011-03-25 12:17:53 -0600296Linux-next has become an integral part of the kernel development process;
297all patches merged during a given merge window should really have found
298their way into linux-next some time before the merge window opens.
Jonathan Corbet75b02142008-09-30 15:15:56 -0600299
Jonathan Corbet75b02142008-09-30 15:15:56 -0600300
Andres Salomonf8306732010-11-18 12:27:36 -08003012.4.1: STAGING TREES
Randy Dunlape6a591e2010-05-23 17:02:30 -0700302
Jonathan Corbet5c050fb2011-03-25 12:17:53 -0600303The kernel source tree contains the drivers/staging/ directory, where
Andres Salomonf8306732010-11-18 12:27:36 -0800304many sub-directories for drivers or filesystems that are on their way to
305being added to the kernel tree live. They remain in drivers/staging while
306they still need more work; once complete, they can be moved into the
307kernel proper. This is a way to keep track of drivers that aren't
308up to Linux kernel coding or quality standards, but people may want to use
309them and track development.
310
Jonathan Corbet5c050fb2011-03-25 12:17:53 -0600311Greg Kroah-Hartman currently maintains the staging tree. Drivers that
312still need work are sent to him, with each driver having its own
313subdirectory in drivers/staging/. Along with the driver source files, a
314TODO file should be present in the directory as well. The TODO file lists
315the pending work that the driver needs for acceptance into the kernel
316proper, as well as a list of people that should be Cc'd for any patches to
317the driver. Current rules require that drivers contributed to staging
318must, at a minimum, compile properly.
319
320Staging can be a relatively easy way to get new drivers into the mainline
321where, with luck, they will come to the attention of other developers and
322improve quickly. Entry into staging is not the end of the story, though;
323code in staging which is not seeing regular progress will eventually be
324removed. Distributors also tend to be relatively reluctant to enable
325staging drivers. So staging is, at best, a stop on the way toward becoming
326a proper mainline driver.
327
Jonathan Corbet75b02142008-09-30 15:15:56 -0600328
3292.5: TOOLS
330
331As can be seen from the above text, the kernel development process depends
332heavily on the ability to herd collections of patches in various
333directions. The whole thing would not work anywhere near as well as it
334does without suitably powerful tools. Tutorials on how to use these tools
335are well beyond the scope of this document, but there is space for a few
336pointers.
337
338By far the dominant source code management system used by the kernel
339community is git. Git is one of a number of distributed version control
340systems being developed in the free software community. It is well tuned
341for kernel development, in that it performs quite well when dealing with
342large repositories and large numbers of patches. It also has a reputation
343for being difficult to learn and use, though it has gotten better over
344time. Some sort of familiarity with git is almost a requirement for kernel
345developers; even if they do not use it for their own work, they'll need git
346to keep up with what other developers (and the mainline) are doing.
347
348Git is now packaged by almost all Linux distributions. There is a home
Randy Dunlapef0eba42010-05-23 17:02:30 -0700349page at:
Jonathan Corbet75b02142008-09-30 15:15:56 -0600350
Randy Dunlapef0eba42010-05-23 17:02:30 -0700351 http://git-scm.com/
Jonathan Corbet75b02142008-09-30 15:15:56 -0600352
Jonathan Corbet5c050fb2011-03-25 12:17:53 -0600353That page has pointers to documentation and tutorials.
Jonathan Corbet75b02142008-09-30 15:15:56 -0600354
355Among the kernel developers who do not use git, the most popular choice is
356almost certainly Mercurial:
357
358 http://www.selenic.com/mercurial/
359
360Mercurial shares many features with git, but it provides an interface which
361many find easier to use.
362
363The other tool worth knowing about is Quilt:
364
365 http://savannah.nongnu.org/projects/quilt/
366
367Quilt is a patch management system, rather than a source code management
368system. It does not track history over time; it is, instead, oriented
369toward tracking a specific set of changes against an evolving code base.
370Some major subsystem maintainers use quilt to manage patches intended to go
371upstream. For the management of certain kinds of trees (-mm, for example),
372quilt is the best tool for the job.
373
374
3752.6: MAILING LISTS
376
377A great deal of Linux kernel development work is done by way of mailing
378lists. It is hard to be a fully-functioning member of the community
379without joining at least one list somewhere. But Linux mailing lists also
380represent a potential hazard to developers, who risk getting buried under a
381load of electronic mail, running afoul of the conventions used on the Linux
382lists, or both.
383
384Most kernel mailing lists are run on vger.kernel.org; the master list can
385be found at:
386
387 http://vger.kernel.org/vger-lists.html
388
389There are lists hosted elsewhere, though; a number of them are at
390lists.redhat.com.
391
392The core mailing list for kernel development is, of course, linux-kernel.
393This list is an intimidating place to be; volume can reach 500 messages per
394day, the amount of noise is high, the conversation can be severely
395technical, and participants are not always concerned with showing a high
396degree of politeness. But there is no other place where the kernel
397development community comes together as a whole; developers who avoid this
398list will miss important information.
399
400There are a few hints which can help with linux-kernel survival:
401
402- Have the list delivered to a separate folder, rather than your main
403 mailbox. One must be able to ignore the stream for sustained periods of
404 time.
405
406- Do not try to follow every conversation - nobody else does. It is
407 important to filter on both the topic of interest (though note that
408 long-running conversations can drift away from the original subject
409 without changing the email subject line) and the people who are
Jonathan Corbet5c050fb2011-03-25 12:17:53 -0600410 participating.
Jonathan Corbet75b02142008-09-30 15:15:56 -0600411
412- Do not feed the trolls. If somebody is trying to stir up an angry
413 response, ignore them.
414
415- When responding to linux-kernel email (or that on other lists) preserve
416 the Cc: header for all involved. In the absence of a strong reason (such
417 as an explicit request), you should never remove recipients. Always make
418 sure that the person you are responding to is in the Cc: list. This
419 convention also makes it unnecessary to explicitly ask to be copied on
420 replies to your postings.
421
422- Search the list archives (and the net as a whole) before asking
423 questions. Some developers can get impatient with people who clearly
424 have not done their homework.
425
426- Avoid top-posting (the practice of putting your answer above the quoted
427 text you are responding to). It makes your response harder to read and
428 makes a poor impression.
429
430- Ask on the correct mailing list. Linux-kernel may be the general meeting
431 point, but it is not the best place to find developers from all
432 subsystems.
433
434The last point - finding the correct mailing list - is a common place for
435beginning developers to go wrong. Somebody who asks a networking-related
436question on linux-kernel will almost certainly receive a polite suggestion
437to ask on the netdev list instead, as that is the list frequented by most
438networking developers. Other lists exist for the SCSI, video4linux, IDE,
439filesystem, etc. subsystems. The best place to look for mailing lists is
440in the MAINTAINERS file packaged with the kernel source.
441
442
4432.7: GETTING STARTED WITH KERNEL DEVELOPMENT
444
445Questions about how to get started with the kernel development process are
446common - from both individuals and companies. Equally common are missteps
447which make the beginning of the relationship harder than it has to be.
448
449Companies often look to hire well-known developers to get a development
450group started. This can, in fact, be an effective technique. But it also
451tends to be expensive and does not do much to grow the pool of experienced
452kernel developers. It is possible to bring in-house developers up to speed
453on Linux kernel development, given the investment of a bit of time. Taking
454this time can endow an employer with a group of developers who understand
455the kernel and the company both, and who can help to train others as well.
456Over the medium term, this is often the more profitable approach.
457
458Individual developers are often, understandably, at a loss for a place to
459start. Beginning with a large project can be intimidating; one often wants
460to test the waters with something smaller first. This is the point where
461some developers jump into the creation of patches fixing spelling errors or
462minor coding style issues. Unfortunately, such patches create a level of
463noise which is distracting for the development community as a whole, so,
464increasingly, they are looked down upon. New developers wishing to
465introduce themselves to the community will not get the sort of reception
466they wish for by these means.
467
468Andrew Morton gives this advice for aspiring kernel developers
469
470 The #1 project for all kernel beginners should surely be "make sure
471 that the kernel runs perfectly at all times on all machines which
472 you can lay your hands on". Usually the way to do this is to work
473 with others on getting things fixed up (this can require
474 persistence!) but that's fine - it's a part of kernel development.
475
476(http://lwn.net/Articles/283982/).
477
478In the absence of obvious problems to fix, developers are advised to look
479at the current lists of regressions and open bugs in general. There is
480never any shortage of issues in need of fixing; by addressing these issues,
481developers will gain experience with the process while, at the same time,
482building respect with the rest of the development community.