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3 Advanced topics
4 ===============
6 At this point, hopefully, you have a handle on how the development process
7 works. There is still more to learn, however! This section will cover a
8 number of topics which can be helpful for developers wanting to become a
9 regular part of the Linux kernel development process.
11 Managing patches with git
12 -------------------------
14 The use of distributed version control for the kernel began in early 2002,
15 when Linus first started playing with the proprietary BitKeeper
16 application. While BitKeeper was controversial, the approach to software
17 version management it embodied most certainly was not. Distributed version
18 control enabled an immediate acceleration of the kernel development
19 project. In current times, there are several free alternatives to
20 BitKeeper. For better or for worse, the kernel project has settled on git
21 as its tool of choice.
23 Managing patches with git can make life much easier for the developer,
24 especially as the volume of those patches grows. Git also has its rough
25 edges and poses certain hazards; it is a young and powerful tool which is
26 still being civilized by its developers. This document will not attempt to
27 teach the reader how to use git; that would be sufficient material for a
28 long document in its own right. Instead, the focus here will be on how git
29 fits into the kernel development process in particular. Developers who
30 wish to come up to speed with git will find more information at:
32 http://git-scm.com/
34 http://www.kernel.org/pub/software/scm/git/docs/user-manual.html
36 and on various tutorials found on the web.
38 The first order of business is to read the above sites and get a solid
39 understanding of how git works before trying to use it to make patches
40 available to others. A git-using developer should be able to obtain a copy
41 of the mainline repository, explore the revision history, commit changes to
42 the tree, use branches, etc. An understanding of git's tools for the
43 rewriting of history (such as rebase) is also useful. Git comes with its
44 own terminology and concepts; a new user of git should know about refs,
45 remote branches, the index, fast-forward merges, pushes and pulls, detached
46 heads, etc. It can all be a little intimidating at the outset, but the
47 concepts are not that hard to grasp with a bit of study.
49 Using git to generate patches for submission by email can be a good
50 exercise while coming up to speed.
52 When you are ready to start putting up git trees for others to look at, you
53 will, of course, need a server that can be pulled from. Setting up such a
54 server with git-daemon is relatively straightforward if you have a system
55 which is accessible to the Internet. Otherwise, free, public hosting sites
56 (Github, for example) are starting to appear on the net. Established
57 developers can get an account on kernel.org, but those are not easy to come
58 by; see http://kernel.org/faq/ for more information.
60 The normal git workflow involves the use of a lot of branches. Each line
61 of development can be separated into a separate "topic branch" and
62 maintained independently. Branches in git are cheap, there is no reason to
63 not make free use of them. And, in any case, you should not do your
64 development in any branch which you intend to ask others to pull from.
65 Publicly-available branches should be created with care; merge in patches
66 from development branches when they are in complete form and ready to go -
67 not before.
69 Git provides some powerful tools which can allow you to rewrite your
70 development history. An inconvenient patch (one which breaks bisection,
71 say, or which has some other sort of obvious bug) can be fixed in place or
72 made to disappear from the history entirely. A patch series can be
73 rewritten as if it had been written on top of today's mainline, even though
74 you have been working on it for months. Changes can be transparently
75 shifted from one branch to another. And so on. Judicious use of git's
76 ability to revise history can help in the creation of clean patch sets with
77 fewer problems.
79 Excessive use of this capability can lead to other problems, though, beyond
80 a simple obsession for the creation of the perfect project history.
81 Rewriting history will rewrite the changes contained in that history,
82 turning a tested (hopefully) kernel tree into an untested one. But, beyond
83 that, developers cannot easily collaborate if they do not have a shared
84 view of the project history; if you rewrite history which other developers
85 have pulled into their repositories, you will make life much more difficult
86 for those developers. So a simple rule of thumb applies here: history
87 which has been exported to others should generally be seen as immutable
88 thereafter.
90 So, once you push a set of changes to your publicly-available server, those
91 changes should not be rewritten. Git will attempt to enforce this rule if
92 you try to push changes which do not result in a fast-forward merge
93 (i.e. changes which do not share the same history). It is possible to
94 override this check, and there may be times when it is necessary to rewrite
95 an exported tree. Moving changesets between trees to avoid conflicts in
96 linux-next is one example. But such actions should be rare. This is one
97 of the reasons why development should be done in private branches (which
98 can be rewritten if necessary) and only moved into public branches when
99 it's in a reasonably advanced state.
101 As the mainline (or other tree upon which a set of changes is based)
102 advances, it is tempting to merge with that tree to stay on the leading
103 edge. For a private branch, rebasing can be an easy way to keep up with
104 another tree, but rebasing is not an option once a tree is exported to the
105 world. Once that happens, a full merge must be done. Merging occasionally
106 makes good sense, but overly frequent merges can clutter the history
107 needlessly. Suggested technique in this case is to merge infrequently, and
108 generally only at specific release points (such as a mainline -rc
109 release). If you are nervous about specific changes, you can always
110 perform test merges in a private branch. The git "rerere" tool can be
111 useful in such situations; it remembers how merge conflicts were resolved
112 so that you don't have to do the same work twice.
114 One of the biggest recurring complaints about tools like git is this: the
115 mass movement of patches from one repository to another makes it easy to
116 slip in ill-advised changes which go into the mainline below the review
117 radar. Kernel developers tend to get unhappy when they see that kind of
118 thing happening; putting up a git tree with unreviewed or off-topic patches
119 can affect your ability to get trees pulled in the future. Quoting Linus:
121 ::
123 You can send me patches, but for me to pull a git patch from you, I
124 need to know that you know what you're doing, and I need to be able
125 to trust things *without* then having to go and check every
126 individual change by hand.
128 (http://lwn.net/Articles/224135/).
130 To avoid this kind of situation, ensure that all patches within a given
131 branch stick closely to the associated topic; a "driver fixes" branch
132 should not be making changes to the core memory management code. And, most
133 importantly, do not use a git tree to bypass the review process. Post an
134 occasional summary of the tree to the relevant list, and, when the time is
135 right, request that the tree be included in linux-next.
137 If and when others start to send patches for inclusion into your tree,
138 don't forget to review them. Also ensure that you maintain the correct
139 authorship information; the git "am" tool does its best in this regard, but
140 you may have to add a "From:" line to the patch if it has been relayed to
141 you via a third party.
143 When requesting a pull, be sure to give all the relevant information: where
144 your tree is, what branch to pull, and what changes will result from the
145 pull. The git request-pull command can be helpful in this regard; it will
146 format the request as other developers expect, and will also check to be
147 sure that you have remembered to push those changes to the public server.
150 Reviewing patches
151 -----------------
153 Some readers will certainly object to putting this section with "advanced
154 topics" on the grounds that even beginning kernel developers should be
155 reviewing patches. It is certainly true that there is no better way to
156 learn how to program in the kernel environment than by looking at code
157 posted by others. In addition, reviewers are forever in short supply; by
158 looking at code you can make a significant contribution to the process as a
159 whole.
161 Reviewing code can be an intimidating prospect, especially for a new kernel
162 developer who may well feel nervous about questioning code - in public -
163 which has been posted by those with more experience. Even code written by
164 the most experienced developers can be improved, though. Perhaps the best
165 piece of advice for reviewers (all reviewers) is this: phrase review
166 comments as questions rather than criticisms. Asking "how does the lock
167 get released in this path?" will always work better than stating "the
168 locking here is wrong."
170 Different developers will review code from different points of view. Some
171 are mostly concerned with coding style and whether code lines have trailing
172 white space. Others will focus primarily on whether the change implemented
173 by the patch as a whole is a good thing for the kernel or not. Yet others
174 will check for problematic locking, excessive stack usage, possible
175 security issues, duplication of code found elsewhere, adequate
176 documentation, adverse effects on performance, user-space ABI changes, etc.
177 All types of review, if they lead to better code going into the kernel, are
178 welcome and worthwhile.