1.2: WHAT THIS DOCUMENT IS ABOUT
-The Linux kernel, at over 6 million lines of code and well over 1000 active
-contributors, is one of the largest and most active free software projects
-in existence. Since its humble beginning in 1991, this kernel has evolved
-into a best-of-breed operating system component which runs on pocket-sized
-digital music players, desktop PCs, the largest supercomputers in
-existence, and all types of systems in between. It is a robust, efficient,
-and scalable solution for almost any situation.
+The Linux kernel, at over 8 million lines of code and well over 1000
+contributors to each release, is one of the largest and most active free
+software projects in existence. Since its humble beginning in 1991, this
+kernel has evolved into a best-of-breed operating system component which
+runs on pocket-sized digital music players, desktop PCs, the largest
+supercomputers in existence, and all types of systems in between. It is a
+robust, efficient, and scalable solution for almost any situation.
With the growth of Linux has come an increase in the number of developers
(and companies) wishing to participate in its development. Hardware
improved by comments from Johannes Berg, James Berry, Alex Chiang, Roland
Dreier, Randy Dunlap, Jake Edge, Jiri Kosina, Matt Mackall, Arthur Marsh,
Amanda McPherson, Andrew Morton, Andrew Price, Tsugikazu Shibata, and
-Jochen Voß.
+Jochen Voß.
This work was supported by the Linux Foundation; thanks especially to
Amanda McPherson, who saw the value of this effort and made it all happen.
- Everything that was said above about code review applies doubly to
closed-source code. Since this code is not available at all, it cannot
have been reviewed by the community and will, beyond doubt, have serious
- problems.
+ problems.
Makers of embedded systems, in particular, may be tempted to disregard much
of what has been said in this section in the belief that they are shipping
major kernel release happening every two or three months. The recent
release history looks like this:
- 2.6.26 July 13, 2008
- 2.6.25 April 16, 2008
- 2.6.24 January 24, 2008
- 2.6.23 October 9, 2007
- 2.6.22 July 8, 2007
- 2.6.21 April 25, 2007
- 2.6.20 February 4, 2007
+ 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 over 10,000
+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.
and staged ahead of time. How that process works will be described in
detail later on).
-The merge window lasts for 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 2.6.26, for
-example, the release which happens at the end of the merge window will be
-called 2.6.26-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.
+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
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 2.6.25 development cycle went (all dates in
-2008):
-
- January 24 2.6.24 stable release
- February 10 2.6.25-rc1, merge window closes
- February 15 2.6.25-rc2
- February 24 2.6.25-rc3
- March 4 2.6.25-rc4
- March 9 2.6.25-rc5
- March 16 2.6.25-rc6
- March 25 2.6.25-rc7
- April 1 2.6.25-rc8
- April 11 2.6.25-rc9
- April 16 2.6.25 stable release
+As an example, here is how the 2.6.38 development cycle went (all dates in
+2011):
+
+ 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
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
-period.
+period.
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
of them are serious.
Once a stable release is made, its ongoing maintenance is passed off to the
-"stable team," currently comprised of Greg Kroah-Hartman and Chris Wright.
-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. Continuing our 2.6.25 example,
-the history (as of this writing) is:
-
- May 1 2.6.25.1
- May 6 2.6.25.2
- May 9 2.6.25.3
- May 15 2.6.25.4
- June 7 2.6.25.5
- June 9 2.6.25.6
- June 16 2.6.25.7
- June 21 2.6.25.8
- June 24 2.6.25.9
-
-Stable updates for a given kernel are made for approximately six months;
-after that, the maintenance of stable releases is solely the responsibility
-of the distributors which have shipped that particular kernel.
+"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 2.6.36.1
+ December 9 2.6.36.2
+ January 7 2.6.36.3
+ February 17 2.6.36.4
+
+2.6.36.4 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
+release.
2.2: THE LIFECYCLE OF A PATCH
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.
+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
2.3: HOW PATCHES GET INTO THE KERNEL
There is exactly one person who can merge patches into the mainline kernel
-repository: Linus Torvalds. But, of the over 12,000 patches which went
-into the 2.6.25 kernel, only 250 (around 2%) were directly chosen by Linus
+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
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
-trust."
+trust."
Clearly, in a system like this, getting patches into the kernel depends on
finding the right maintainer. Sending patches directly to Linus is not
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.
+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
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 10% of the patches going into the mainline
-get there via -mm.
+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:
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 other -next tree, started more recently, is linux-next, maintained by
+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
http://linux.f-seidel.de/linux-next/pmwiki/
-How the linux-next tree will fit into the development process is still
-changing. As of this writing, the first full development cycle involving
-linux-next (2.6.26) is coming to an end; thus far, it has proved to be a
-valuable resource for finding and fixing integration problems before the
-beginning of the merge window. See http://lwn.net/Articles/287155/ for
-more information on how linux-next has worked to set up the 2.6.27 merge
-window.
-
-Some developers have begun to suggest that linux-next should be used as the
-target for future development as well. The linux-next tree does tend to be
-far ahead of the mainline and is more representative of the tree into which
-any new work will be merged. The downside to this idea is that the
-volatility of linux-next tends to make it a difficult development target.
-See http://lwn.net/Articles/289013/ for more information on this topic, and
-stay tuned; much is still in flux where linux-next is involved.
+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.
+
2.4.1: STAGING TREES
-The kernel source tree now contains the drivers/staging/ directory, where
+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
up to Linux kernel coding or quality standards, but people may want to use
them and track development.
-Greg Kroah-Hartman currently (as of 2.6.36) 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. Staging drivers that don't currently build should
-have their config entries depend upon CONFIG_BROKEN. Once they can
-be successfully built without outside patches, CONFIG_BROKEN can be removed.
+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
http://git-scm.com/
-That page has pointers to documentation and tutorials. One should be
-aware, in particular, of the Kernel Hacker's Guide to git, which has
-information specific to kernel development:
-
- http://linux.yyz.us/git-howto.html
+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:
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.
+ participating.
- Do not feed the trolls. If somebody is trying to stir up an angry
response, ignore them.
- The AppArmor security module made use of internal virtual filesystem
data structures in ways which were considered to be unsafe and
- unreliable. This code has since been significantly reworked, but
- remains outside of the mainline.
+ unreliable. This concern (among others) kept AppArmor out of the
+ mainline for years.
In each of these cases, a great deal of pain and extra work could have been
avoided with some early discussion with the kernel developers.
patches. Those are the people who will be best placed to help with a new
development project.
+The task of finding the right maintainer is sometimes challenging enough
+that the kernel developers have added a script to ease the process:
+
+ .../scripts/get_maintainer.pl
+
+This script will return the current maintainer(s) for a given file or
+directory when given the "-f" option. If passed a patch on the
+command line, it will list the maintainers who should probably receive
+copies of the patch. There are a number of options regulating how hard
+get_maintainer.pl will search for maintainers; please be careful about
+using the more aggressive options as you may end up including developers
+who have no real interest in the code you are modifying.
+
If all else fails, talking to Andrew Morton can be an effective way to
track down a maintainer for a specific piece of code.
matter is (1) kernel developers tend to be busy, (2) there is no shortage
of people with grand plans and little code (or even prospect of code) to
back them up, and (3) nobody is obligated to review or comment on ideas
-posted by others. If a request-for-comments posting yields little in the
-way of comments, do not assume that it means there is no interest in the
-project. Unfortunately, you also cannot assume that there are no problems
-with your idea. The best thing to do in this situation is to proceed,
-keeping the community informed as you go.
+posted by others. Beyond that, high-level designs often hide problems
+which are only reviewed when somebody actually tries to implement those
+designs; for that reason, kernel developers would rather see the code.
+
+If a request-for-comments posting yields little in the way of comments, do
+not assume that it means there is no interest in the project.
+Unfortunately, you also cannot assume that there are no problems with your
+idea. The best thing to do in this situation is to proceed, keeping the
+community informed as you go.
3.5: GETTING OFFICIAL BUY-IN
often does not apply to contemporary hardware. Space *is* time, in that a
larger program will run slower than one which is more compact.
+More recent compilers take an increasingly active role in deciding whether
+a given function should actually be inlined or not. So the liberal
+placement of "inline" keywords may not just be excessive; it could also be
+irrelevant.
+
* Locking
distributor does not package it); it can then be run on the code by adding
"C=1" to your make command.
+The "Coccinelle" tool (http://coccinelle.lip6.fr/) is able to find a wide
+variety of potential coding problems; it can also propose fixes for those
+problems. Quite a few "semantic patches" for the kernel have been packaged
+under the scripts/coccinelle directory; running "make coccicheck" will run
+through those semantic patches and report on any problems found. See
+Documentation/coccinelle.txt for more information.
+
Other kinds of portability errors are best found by compiling your code for
other architectures. If you do not happen to have an S/390 system or a
Blackfin development board handy, you can still perform the compilation
changelog. Log entries should describe the problem being solved, the form
of the solution, the people who worked on the patch, any relevant
effects on performance, and anything else that might be needed to
-understand the patch.
+understand the patch. Be sure that the changelog says *why* the patch is
+worth applying; a surprising number of developers fail to provide that
+information.
Any code which adds a new user-space interface - including new sysfs or
/proc files - should include documentation of that interface which enables
appropriate entries to this file.
Any new configuration options must be accompanied by help text which
-clearly explains the options and when the user might want to select them.
+clearly explains the options and when the user might want to select them.
Internal API information for many subsystems is documented by way of
specially-formatted comments; these comments can be extracted and formatted
lead to literally hundreds or thousands of changes - many of which are
likely to conflict with work being done by other developers. Needless to
say, this can be a large job, so it is best to be sure that the
-justification is solid.
+justification is solid. Note that the Coccinelle tool can help with
+wide-ranging API changes.
When making an incompatible API change, one should, whenever possible,
ensure that code which has not been updated is caught by the compiler.
Patches must be prepared against a specific version of the kernel. As a
general rule, a patch should be based on the current mainline as found in
-Linus's git tree. It may become necessary to make versions against -mm,
-linux-next, or a subsystem tree, though, to facilitate wider testing and
-review. Depending on the area of your patch and what is going on
-elsewhere, basing a patch against these other trees can require a
-significant amount of work resolving conflicts and dealing with API
-changes.
+Linus's git tree. When basing on mainline, start with a well-known release
+point - a stable or -rc release - rather than branching off the mainline at
+an arbitrary spot.
+
+It may become necessary to make versions against -mm, linux-next, or a
+subsystem tree, though, to facilitate wider testing and review. Depending
+on the area of your patch and what is going on elsewhere, basing a patch
+against these other trees can require a significant amount of work
+resolving conflicts and dealing with API changes.
Only the most simple changes should be formatted as a single patch;
everything else should be made as a logical series of changes. Splitting
result is a broken kernel, you will make life harder for developers and
users who are engaging in the noble work of tracking down problems.
- - Do not overdo it, though. One developer recently posted a set of edits
+ - Do not overdo it, though. One developer once posted a set of edits
to a single file as 500 separate patches - an act which did not make him
the most popular person on the kernel mailing list. A single patch can
be reasonably large as long as it still contains a single *logical*
- change.
+ change.
- It can be tempting to add a whole new infrastructure with a series of
patches, but to leave that infrastructure unused until the final patch
for the change as well as possible given the one-line constraint. The
detailed description can then amplify on those topics and provide any
needed additional information. If the patch fixes a bug, cite the commit
-which introduced the bug if possible. If a problem is associated with
+which introduced the bug if possible (and please provide both the commit ID
+and the title when citing commits). If a problem is associated with
specific log or compiler output, include that output to help others
searching for a solution to the same problem. If the change is meant to
support other changes coming in later patch, say so. If internal APIs are
which have had gratuitous white-space changes or line wrapping performed
by the mail client will not apply at the other end, and often will not
be examined in any detail. If there is any doubt at all, mail the patch
- to yourself and convince yourself that it shows up intact.
+ to yourself and convince yourself that it shows up intact.
Documentation/email-clients.txt has some helpful hints on making
specific mail clients work for sending patches.
where "nn" is the ordinal number of the patch, "mm" is the total number of
patches in the series, and "subsys" is the name of the affected subsystem.
-Clearly, nn/mm can be omitted for a single, standalone patch.
+Clearly, nn/mm can be omitted for a single, standalone patch.
If you have a significant series of patches, it is customary to send an
introductory description as part zero. This convention is not universally
sent as a reply to the first part so that they all thread together at the
receiving end. Tools like git and quilt have commands to mail out a set of
patches with the proper threading. If you have a long series, though, and
-are using git, please provide the --no-chain-reply-to option to avoid
+are using git, please stay away from the --chain-reply-to option to avoid
creating exceptionally deep nesting.
that you don't realize that something is fundamentally wrong or, perhaps,
you're not even solving the right problem.
+Andrew Morton has suggested that every review comment which does not result
+in a code change should result in an additional code comment instead; that
+can help future reviewers avoid the questions which came up the first time
+around.
+
One fatal mistake is to ignore review comments in the hope that they will
go away. They will not go away. If you repost code without having
responded to the comments you got the time before, you're likely to find
subsystem to the next; each maintainer has his or her own way of doing
things. In particular, there may be more than one tree - one, perhaps,
dedicated to patches planned for the next merge window, and another for
-longer-term work.
+longer-term work.
For patches applying to areas for which there is no obvious subsystem tree
(memory management patches, for example), the default tree often ends up
Inclusion into a subsystem tree can bring a higher level of visibility to a
patch. Now other developers working with that tree will get the patch by
-default. Subsystem trees typically feed into -mm and linux-next as well,
-making their contents visible to the development community as a whole. At
-this point, there's a good chance that you will get more comments from a
-new set of reviewers; these comments need to be answered as in the previous
-round.
+default. Subsystem trees typically feed linux-next as well, making their
+contents visible to the development community as a whole. At this point,
+there's a good chance that you will get more comments from a new set of
+reviewers; these comments need to be answered as in the previous round.
What may also happen at this point, depending on the nature of your patch,
is that conflicts with work being done by others turn up. In the worst
to trust things *without* then having to go and check every
individual change by hand.
-(http://lwn.net/Articles/224135/).
+(http://lwn.net/Articles/224135/).
To avoid this kind of situation, ensure that all patches within a given
branch stick closely to the associated topic; a "driver fixes" branch
your tree is, what branch to pull, and what changes will result from the
pull. The git request-pull command can be helpful in this regard; it will
format the request as other developers expect, and will also check to be
-sure that you have remembered to push those changes to the public server.
+sure that you have remembered to push those changes to the public server.
7.2: REVIEWING PATCHES
git.openpandora.org / pandora-kernel.git / commitdiff