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Based on kernel version 3.15.4. Page generated on 2014-07-07 09:00 EST.

1	7: ADVANCED TOPICS
2	
3	At this point, hopefully, you have a handle on how the development process
4	works.  There is still more to learn, however!  This section will cover a
5	number of topics which can be helpful for developers wanting to become a
6	regular part of the Linux kernel development process.
7	
8	7.1: MANAGING PATCHES WITH GIT
9	
10	The use of distributed version control for the kernel began in early 2002,
11	when Linus first started playing with the proprietary BitKeeper
12	application.  While BitKeeper was controversial, the approach to software
13	version management it embodied most certainly was not.  Distributed version
14	control enabled an immediate acceleration of the kernel development
15	project.  In current times, there are several free alternatives to
16	BitKeeper.  For better or for worse, the kernel project has settled on git
17	as its tool of choice.
18	
19	Managing patches with git can make life much easier for the developer,
20	especially as the volume of those patches grows.  Git also has its rough
21	edges and poses certain hazards; it is a young and powerful tool which is
22	still being civilized by its developers.  This document will not attempt to
23	teach the reader how to use git; that would be sufficient material for a
24	long document in its own right.  Instead, the focus here will be on how git
25	fits into the kernel development process in particular.  Developers who
26	wish to come up to speed with git will find more information at:
27	
28		http://git-scm.com/
29	
30		http://www.kernel.org/pub/software/scm/git/docs/user-manual.html
31	
32	and on various tutorials found on the web.
33	
34	The first order of business is to read the above sites and get a solid
35	understanding of how git works before trying to use it to make patches
36	available to others.  A git-using developer should be able to obtain a copy
37	of the mainline repository, explore the revision history, commit changes to
38	the tree, use branches, etc.  An understanding of git's tools for the
39	rewriting of history (such as rebase) is also useful.  Git comes with its
40	own terminology and concepts; a new user of git should know about refs,
41	remote branches, the index, fast-forward merges, pushes and pulls, detached
42	heads, etc.  It can all be a little intimidating at the outset, but the
43	concepts are not that hard to grasp with a bit of study.
44	
45	Using git to generate patches for submission by email can be a good
46	exercise while coming up to speed.
47	
48	When you are ready to start putting up git trees for others to look at, you
49	will, of course, need a server that can be pulled from.  Setting up such a
50	server with git-daemon is relatively straightforward if you have a system
51	which is accessible to the Internet.  Otherwise, free, public hosting sites
52	(Github, for example) are starting to appear on the net.  Established
53	developers can get an account on kernel.org, but those are not easy to come
54	by; see http://kernel.org/faq/ for more information.
55	
56	The normal git workflow involves the use of a lot of branches.  Each line
57	of development can be separated into a separate "topic branch" and
58	maintained independently.  Branches in git are cheap, there is no reason to
59	not make free use of them.  And, in any case, you should not do your
60	development in any branch which you intend to ask others to pull from.
61	Publicly-available branches should be created with care; merge in patches
62	from development branches when they are in complete form and ready to go -
63	not before.
64	
65	Git provides some powerful tools which can allow you to rewrite your
66	development history.  An inconvenient patch (one which breaks bisection,
67	say, or which has some other sort of obvious bug) can be fixed in place or
68	made to disappear from the history entirely.  A patch series can be
69	rewritten as if it had been written on top of today's mainline, even though
70	you have been working on it for months.  Changes can be transparently
71	shifted from one branch to another.  And so on.  Judicious use of git's
72	ability to revise history can help in the creation of clean patch sets with
73	fewer problems.
74	
75	Excessive use of this capability can lead to other problems, though, beyond
76	a simple obsession for the creation of the perfect project history.
77	Rewriting history will rewrite the changes contained in that history,
78	turning a tested (hopefully) kernel tree into an untested one.  But, beyond
79	that, developers cannot easily collaborate if they do not have a shared
80	view of the project history; if you rewrite history which other developers
81	have pulled into their repositories, you will make life much more difficult
82	for those developers.  So a simple rule of thumb applies here: history
83	which has been exported to others should generally be seen as immutable
84	thereafter.
85	
86	So, once you push a set of changes to your publicly-available server, those
87	changes should not be rewritten.  Git will attempt to enforce this rule if
88	you try to push changes which do not result in a fast-forward merge
89	(i.e. changes which do not share the same history).  It is possible to
90	override this check, and there may be times when it is necessary to rewrite
91	an exported tree.  Moving changesets between trees to avoid conflicts in
92	linux-next is one example.  But such actions should be rare.  This is one
93	of the reasons why development should be done in private branches (which
94	can be rewritten if necessary) and only moved into public branches when
95	it's in a reasonably advanced state.
96	
97	As the mainline (or other tree upon which a set of changes is based)
98	advances, it is tempting to merge with that tree to stay on the leading
99	edge.  For a private branch, rebasing can be an easy way to keep up with
100	another tree, but rebasing is not an option once a tree is exported to the
101	world.  Once that happens, a full merge must be done.  Merging occasionally
102	makes good sense, but overly frequent merges can clutter the history
103	needlessly.  Suggested technique in this case is to merge infrequently, and
104	generally only at specific release points (such as a mainline -rc
105	release).  If you are nervous about specific changes, you can always
106	perform test merges in a private branch.  The git "rerere" tool can be
107	useful in such situations; it remembers how merge conflicts were resolved
108	so that you don't have to do the same work twice.
109	
110	One of the biggest recurring complaints about tools like git is this: the
111	mass movement of patches from one repository to another makes it easy to
112	slip in ill-advised changes which go into the mainline below the review
113	radar.  Kernel developers tend to get unhappy when they see that kind of
114	thing happening; putting up a git tree with unreviewed or off-topic patches
115	can affect your ability to get trees pulled in the future.  Quoting Linus:
116	
117		You can send me patches, but for me to pull a git patch from you, I
118		need to know that you know what you're doing, and I need to be able
119		to trust things *without* then having to go and check every
120		individual change by hand.
121	
122	(http://lwn.net/Articles/224135/).
123	
124	To avoid this kind of situation, ensure that all patches within a given
125	branch stick closely to the associated topic; a "driver fixes" branch
126	should not be making changes to the core memory management code.  And, most
127	importantly, do not use a git tree to bypass the review process.  Post an
128	occasional summary of the tree to the relevant list, and, when the time is
129	right, request that the tree be included in linux-next.
130	
131	If and when others start to send patches for inclusion into your tree,
132	don't forget to review them.  Also ensure that you maintain the correct
133	authorship information; the git "am" tool does its best in this regard, but
134	you may have to add a "From:" line to the patch if it has been relayed to
135	you via a third party.
136	
137	When requesting a pull, be sure to give all the relevant information: where
138	your tree is, what branch to pull, and what changes will result from the
139	pull.  The git request-pull command can be helpful in this regard; it will
140	format the request as other developers expect, and will also check to be
141	sure that you have remembered to push those changes to the public server.
142	
143	
144	7.2: REVIEWING PATCHES
145	
146	Some readers will certainly object to putting this section with "advanced
147	topics" on the grounds that even beginning kernel developers should be
148	reviewing patches.  It is certainly true that there is no better way to
149	learn how to program in the kernel environment than by looking at code
150	posted by others.  In addition, reviewers are forever in short supply; by
151	looking at code you can make a significant contribution to the process as a
152	whole.
153	
154	Reviewing code can be an intimidating prospect, especially for a new kernel
155	developer who may well feel nervous about questioning code - in public -
156	which has been posted by those with more experience.  Even code written by
157	the most experienced developers can be improved, though.  Perhaps the best
158	piece of advice for reviewers (all reviewers) is this: phrase review
159	comments as questions rather than criticisms.  Asking "how does the lock
160	get released in this path?" will always work better than stating "the
161	locking here is wrong."
162	
163	Different developers will review code from different points of view.  Some
164	are mostly concerned with coding style and whether code lines have trailing
165	white space.  Others will focus primarily on whether the change implemented
166	by the patch as a whole is a good thing for the kernel or not.  Yet others
167	will check for problematic locking, excessive stack usage, possible
168	security issues, duplication of code found elsewhere, adequate
169	documentation, adverse effects on performance, user-space ABI changes, etc.
170	All types of review, if they lead to better code going into the kernel, are
171	welcome and worthwhile.
172	
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