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-7: ADVANCED TOPICS
-
-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.
-
-7.1: 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:
-
- http://git-scm.com/
-
- http://www.kernel.org/pub/software/scm/git/docs/user-manual.html
-
-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 -
-not before.
-
-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
-fewer problems.
-
-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
-thereafter.
-
-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.
-
-(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
-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.
-
-
-7.2: REVIEWING PATCHES
-
-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
-whole.
-
-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.
-
-