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 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.
 
 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:
 
         https://git-scm.com/
 
         https://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 https://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.
 
 (https://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.
 
 
 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.

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