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1 .. SPDX-License-Identifier: GPL-2.0
3 ====================
4 Rebasing and merging
5 ====================
7 Maintaining a subsystem, as a general rule, requires a familiarity with the
8 Git source-code management system. Git is a powerful tool with a lot of
9 features; as is often the case with such tools, there are right and wrong
10 ways to use those features. This document looks in particular at the use
11 of rebasing and merging. Maintainers often get in trouble when they use
12 those tools incorrectly, but avoiding problems is not actually all that
13 hard.
15 One thing to be aware of in general is that, unlike many other projects,
16 the kernel community is not scared by seeing merge commits in its
17 development history. Indeed, given the scale of the project, avoiding
18 merges would be nearly impossible. Some problems encountered by
19 maintainers result from a desire to avoid merges, while others come from
20 merging a little too often.
22 Rebasing
23 ========
25 "Rebasing" is the process of changing the history of a series of commits
26 within a repository. There are two different types of operations that are
27 referred to as rebasing since both are done with the ``git rebase``
28 command, but there are significant differences between them:
30 - Changing the parent (starting) commit upon which a series of patches is
31 built. For example, a rebase operation could take a patch set built on
32 the previous kernel release and base it, instead, on the current
33 release. We'll call this operation "reparenting" in the discussion
34 below.
36 - Changing the history of a set of patches by fixing (or deleting) broken
37 commits, adding patches, adding tags to commit changelogs, or changing
38 the order in which commits are applied. In the following text, this
39 type of operation will be referred to as "history modification"
41 The term "rebasing" will be used to refer to both of the above operations.
42 Used properly, rebasing can yield a cleaner and clearer development
43 history; used improperly, it can obscure that history and introduce bugs.
45 There are a few rules of thumb that can help developers to avoid the worst
46 perils of rebasing:
48 - History that has been exposed to the world beyond your private system
49 should usually not be changed. Others may have pulled a copy of your
50 tree and built on it; modifying your tree will create pain for them. If
51 work is in need of rebasing, that is usually a sign that it is not yet
52 ready to be committed to a public repository.
54 That said, there are always exceptions. Some trees (linux-next being
55 a significant example) are frequently rebased by their nature, and
56 developers know not to base work on them. Developers will sometimes
57 expose an unstable branch for others to test with or for automated
58 testing services. If you do expose a branch that may be unstable in
59 this way, be sure that prospective users know not to base work on it.
61 - Do not rebase a branch that contains history created by others. If you
62 have pulled changes from another developer's repository, you are now a
63 custodian of their history. You should not change it. With few
64 exceptions, for example, a broken commit in a tree like this should be
65 explicitly reverted rather than disappeared via history modification.
67 - Do not reparent a tree without a good reason to do so. Just being on a
68 newer base or avoiding a merge with an upstream repository is not
69 generally a good reason.
71 - If you must reparent a repository, do not pick some random kernel commit
72 as the new base. The kernel is often in a relatively unstable state
73 between release points; basing development on one of those points
74 increases the chances of running into surprising bugs. When a patch
75 series must move to a new base, pick a stable point (such as one of
76 the -rc releases) to move to.
78 - Realize that reparenting a patch series (or making significant history
79 modifications) changes the environment in which it was developed and,
80 likely, invalidates much of the testing that was done. A reparented
81 patch series should, as a general rule, be treated like new code and
82 retested from the beginning.
84 A frequent cause of merge-window trouble is when Linus is presented with a
85 patch series that has clearly been reparented, often to a random commit,
86 shortly before the pull request was sent. The chances of such a series
87 having been adequately tested are relatively low - as are the chances of
88 the pull request being acted upon.
90 If, instead, rebasing is limited to private trees, commits are based on a
91 well-known starting point, and they are well tested, the potential for
92 trouble is low.
94 Merging
95 =======
97 Merging is a common operation in the kernel development process; the 5.1
98 development cycle included 1,126 merge commits - nearly 9% of the total.
99 Kernel work is accumulated in over 100 different subsystem trees, each of
100 which may contain multiple topic branches; each branch is usually developed
101 independently of the others. So naturally, at least one merge will be
102 required before any given branch finds its way into an upstream repository.
104 Many projects require that branches in pull requests be based on the
105 current trunk so that no merge commits appear in the history. The kernel
106 is not such a project; any rebasing of branches to avoid merges will, most
107 likely, lead to trouble.
109 Subsystem maintainers find themselves having to do two types of merges:
110 from lower-level subsystem trees and from others, either sibling trees or
111 the mainline. The best practices to follow differ in those two situations.
113 Merging from lower-level trees
114 ------------------------------
116 Larger subsystems tend to have multiple levels of maintainers, with the
117 lower-level maintainers sending pull requests to the higher levels. Acting
118 on such a pull request will almost certainly generate a merge commit; that
119 is as it should be. In fact, subsystem maintainers may want to use
120 the --no-ff flag to force the addition of a merge commit in the rare cases
121 where one would not normally be created so that the reasons for the merge
122 can be recorded. The changelog for the merge should, for any kind of
123 merge, say *why* the merge is being done. For a lower-level tree, "why" is
124 usually a summary of the changes that will come with that pull.
126 Maintainers at all levels should be using signed tags on their pull
127 requests, and upstream maintainers should verify the tags when pulling
128 branches. Failure to do so threatens the security of the development
129 process as a whole.
131 As per the rules outlined above, once you have merged somebody else's
132 history into your tree, you cannot rebase that branch, even if you
133 otherwise would be able to.
135 Merging from sibling or upstream trees
136 --------------------------------------
138 While merges from downstream are common and unremarkable, merges from other
139 trees tend to be a red flag when it comes time to push a branch upstream.
140 Such merges need to be carefully thought about and well justified, or
141 there's a good chance that a subsequent pull request will be rejected.
143 It is natural to want to merge the master branch into a repository; this
144 type of merge is often called a "back merge". Back merges can help to make
145 sure that there are no conflicts with parallel development and generally
146 gives a warm, fuzzy feeling of being up-to-date. But this temptation
147 should be avoided almost all of the time.
149 Why is that? Back merges will muddy the development history of your own
150 branch. They will significantly increase your chances of encountering bugs
151 from elsewhere in the community and make it hard to ensure that the work
152 you are managing is stable and ready for upstream. Frequent merges can
153 also obscure problems with the development process in your tree; they can
154 hide interactions with other trees that should not be happening (often) in
155 a well-managed branch.
157 That said, back merges are occasionally required; when that happens, be
158 sure to document *why* it was required in the commit message. As always,
159 merge to a well-known stable point, rather than to some random commit.
160 Even then, you should not back merge a tree above your immediate upstream
161 tree; if a higher-level back merge is really required, the upstream tree
162 should do it first.
164 One of the most frequent causes of merge-related trouble is when a
165 maintainer merges with the upstream in order to resolve merge conflicts
166 before sending a pull request. Again, this temptation is easy enough to
167 understand, but it should absolutely be avoided. This is especially true
168 for the final pull request: Linus is adamant that he would much rather see
169 merge conflicts than unnecessary back merges. Seeing the conflicts lets
170 him know where potential problem areas are. He does a lot of merges (382
171 in the 5.1 development cycle) and has gotten quite good at conflict
172 resolution - often better than the developers involved.
174 So what should a maintainer do when there is a conflict between their
175 subsystem branch and the mainline? The most important step is to warn
176 Linus in the pull request that the conflict will happen; if nothing else,
177 that demonstrates an awareness of how your branch fits into the whole. For
178 especially difficult conflicts, create and push a *separate* branch to show
179 how you would resolve things. Mention that branch in your pull request,
180 but the pull request itself should be for the unmerged branch.
182 Even in the absence of known conflicts, doing a test merge before sending a
183 pull request is a good idea. It may alert you to problems that you somehow
184 didn't see from linux-next and helps to understand exactly what you are
185 asking upstream to do.
187 Another reason for doing merges of upstream or another subsystem tree is to
188 resolve dependencies. These dependency issues do happen at times, and
189 sometimes a cross-merge with another tree is the best way to resolve them;
190 as always, in such situations, the merge commit should explain why the
191 merge has been done. Take a moment to do it right; people will read those
192 changelogs.
194 Often, though, dependency issues indicate that a change of approach is
195 needed. Merging another subsystem tree to resolve a dependency risks
196 bringing in other bugs and should almost never be done. If that subsystem
197 tree fails to be pulled upstream, whatever problems it had will block the
198 merging of your tree as well. Preferable alternatives include agreeing
199 with the maintainer to carry both sets of changes in one of the trees or
200 creating a topic branch dedicated to the prerequisite commits that can be
201 merged into both trees. If the dependency is related to major
202 infrastructural changes, the right solution might be to hold the dependent
203 commits for one development cycle so that those changes have time to
204 stabilize in the mainline.
206 Finally
207 =======
209 It is relatively common to merge with the mainline toward the beginning of
210 the development cycle in order to pick up changes and fixes done elsewhere
211 in the tree. As always, such a merge should pick a well-known release
212 point rather than some random spot. If your upstream-bound branch has
213 emptied entirely into the mainline during the merge window, you can pull it
214 forward with a command like::
216 git merge v5.2-rc1^0
218 The "^0" will cause Git to do a fast-forward merge (which should be
219 possible in this situation), thus avoiding the addition of a spurious merge
220 commit.
222 The guidelines laid out above are just that: guidelines. There will always
223 be situations that call out for a different solution, and these guidelines
224 should not prevent developers from doing the right thing when the need
225 arises. But one should always think about whether the need has truly
226 arisen and be prepared to explain why something abnormal needs to be done.