|At this point, hopefully, you have a handle on how the development process
|works. There is still more to learn, however! This section will cover a
|number of topics which can be helpful for developers wanting to become a
|regular part of the Linux kernel development process.
|Managing patches with git
|The use of distributed version control for the kernel began in early 2002,
|when Linus first started playing with the proprietary BitKeeper
|application. While BitKeeper was controversial, the approach to software
|version management it embodied most certainly was not. Distributed version
|control enabled an immediate acceleration of the kernel development
|project. In current times, there are several free alternatives to
|BitKeeper. For better or for worse, the kernel project has settled on git
|as its tool of choice.
|Managing patches with git can make life much easier for the developer,
|especially as the volume of those patches grows. Git also has its rough
|edges and poses certain hazards; it is a young and powerful tool which is
|still being civilized by its developers. This document will not attempt to
|teach the reader how to use git; that would be sufficient material for a
|long document in its own right. Instead, the focus here will be on how git
|fits into the kernel development process in particular. Developers who
|wish to come up to speed with git will find more information at:
|and on various tutorials found on the web.
|The first order of business is to read the above sites and get a solid
|understanding of how git works before trying to use it to make patches
|available to others. A git-using developer should be able to obtain a copy
|of the mainline repository, explore the revision history, commit changes to
|the tree, use branches, etc. An understanding of git's tools for the
|rewriting of history (such as rebase) is also useful. Git comes with its
|own terminology and concepts; a new user of git should know about refs,
|remote branches, the index, fast-forward merges, pushes and pulls, detached
|heads, etc. It can all be a little intimidating at the outset, but the
|concepts are not that hard to grasp with a bit of study.
|Using git to generate patches for submission by email can be a good
|exercise while coming up to speed.
|When you are ready to start putting up git trees for others to look at, you
|will, of course, need a server that can be pulled from. Setting up such a
|server with git-daemon is relatively straightforward if you have a system
|which is accessible to the Internet. Otherwise, free, public hosting sites
|(Github, for example) are starting to appear on the net. Established
|developers can get an account on kernel.org, but those are not easy to come
|by; see http://kernel.org/faq/ for more information.
|The normal git workflow involves the use of a lot of branches. Each line
|of development can be separated into a separate "topic branch" and
|maintained independently. Branches in git are cheap, there is no reason to
|not make free use of them. And, in any case, you should not do your
|development in any branch which you intend to ask others to pull from.
|Publicly-available branches should be created with care; merge in patches
|from development branches when they are in complete form and ready to go -
|Git provides some powerful tools which can allow you to rewrite your
|development history. An inconvenient patch (one which breaks bisection,
|say, or which has some other sort of obvious bug) can be fixed in place or
|made to disappear from the history entirely. A patch series can be
|rewritten as if it had been written on top of today's mainline, even though
|you have been working on it for months. Changes can be transparently
|shifted from one branch to another. And so on. Judicious use of git's
|ability to revise history can help in the creation of clean patch sets with
|Excessive use of this capability can lead to other problems, though, beyond
|a simple obsession for the creation of the perfect project history.
|Rewriting history will rewrite the changes contained in that history,
|turning a tested (hopefully) kernel tree into an untested one. But, beyond
|that, developers cannot easily collaborate if they do not have a shared
|view of the project history; if you rewrite history which other developers
|have pulled into their repositories, you will make life much more difficult
|for those developers. So a simple rule of thumb applies here: history
|which has been exported to others should generally be seen as immutable
|So, once you push a set of changes to your publicly-available server, those
|changes should not be rewritten. Git will attempt to enforce this rule if
|you try to push changes which do not result in a fast-forward merge
|(i.e. changes which do not share the same history). It is possible to
|override this check, and there may be times when it is necessary to rewrite
|an exported tree. Moving changesets between trees to avoid conflicts in
|linux-next is one example. But such actions should be rare. This is one
|of the reasons why development should be done in private branches (which
|can be rewritten if necessary) and only moved into public branches when
|it's in a reasonably advanced state.
|As the mainline (or other tree upon which a set of changes is based)
|advances, it is tempting to merge with that tree to stay on the leading
|edge. For a private branch, rebasing can be an easy way to keep up with
|another tree, but rebasing is not an option once a tree is exported to the
|world. Once that happens, a full merge must be done. Merging occasionally
|makes good sense, but overly frequent merges can clutter the history
|needlessly. Suggested technique in this case is to merge infrequently, and
|generally only at specific release points (such as a mainline -rc
|release). If you are nervous about specific changes, you can always
|perform test merges in a private branch. The git "rerere" tool can be
|useful in such situations; it remembers how merge conflicts were resolved
|so that you don't have to do the same work twice.
|One of the biggest recurring complaints about tools like git is this: the
|mass movement of patches from one repository to another makes it easy to
|slip in ill-advised changes which go into the mainline below the review
|radar. Kernel developers tend to get unhappy when they see that kind of
|thing happening; putting up a git tree with unreviewed or off-topic patches
|can affect your ability to get trees pulled in the future. Quoting Linus:
| You can send me patches, but for me to pull a git patch from you, I
| need to know that you know what you're doing, and I need to be able
| to trust things *without* then having to go and check every
| individual change by hand.
|To avoid this kind of situation, ensure that all patches within a given
|branch stick closely to the associated topic; a "driver fixes" branch
|should not be making changes to the core memory management code. And, most
|importantly, do not use a git tree to bypass the review process. Post an
|occasional summary of the tree to the relevant list, and, when the time is
|right, request that the tree be included in linux-next.
|If and when others start to send patches for inclusion into your tree,
|don't forget to review them. Also ensure that you maintain the correct
|authorship information; the git "am" tool does its best in this regard, but
|you may have to add a "From:" line to the patch if it has been relayed to
|you via a third party.
|When requesting a pull, be sure to give all the relevant information: where
|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.
|Some readers will certainly object to putting this section with "advanced
|topics" on the grounds that even beginning kernel developers should be
|reviewing patches. It is certainly true that there is no better way to
|learn how to program in the kernel environment than by looking at code
|posted by others. In addition, reviewers are forever in short supply; by
|looking at code you can make a significant contribution to the process as a
|Reviewing code can be an intimidating prospect, especially for a new kernel
|developer who may well feel nervous about questioning code - in public -
|which has been posted by those with more experience. Even code written by
|the most experienced developers can be improved, though. Perhaps the best
|piece of advice for reviewers (all reviewers) is this: phrase review
|comments as questions rather than criticisms. Asking "how does the lock
|get released in this path?" will always work better than stating "the
|locking here is wrong."
|Different developers will review code from different points of view. Some
|are mostly concerned with coding style and whether code lines have trailing
|white space. Others will focus primarily on whether the change implemented
|by the patch as a whole is a good thing for the kernel or not. Yet others
|will check for problematic locking, excessive stack usage, possible
|security issues, duplication of code found elsewhere, adequate
|documentation, adverse effects on performance, user-space ABI changes, etc.
|All types of review, if they lead to better code going into the kernel, are
|welcome and worthwhile.