1 .. SPDX-License-Identifier: GPL-2.0 2 3 ==================== 4 Rebasing and merging 5 ==================== 6 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. 14 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. 21 22 Rebasing 23 ======== 24 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: 29 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. 35 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" 40 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. 44 45 There are a few rules of thumb that can help developers to avoid the worst 46 perils of rebasing: 47 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. 53 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. 60 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. 66 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. 70 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. 77 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. 83 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. 89 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. 93 94 Merging 95 ======= 96 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. 103 104 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. 108 109 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. 112 113 Merging from lower-level trees 114 ------------------------------ 115 116 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. 125 126 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. 130 131 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. 134 135 Merging from sibling or upstream trees 136 -------------------------------------- 137 138 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. 142 143 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. 148 149 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. 156 157 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. 163 164 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. 173 174 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. 181 182 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. 186 187 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. 193 194 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. 205 206 Finally 207 ======= 208 209 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:: 215 216 git merge --ff-only v5.2-rc1 217 218 The guidelines laid out above are just that: guidelines. There will always 219 be situations that call out for a different solution, and these guidelines 220 should not prevent developers from doing the right thing when the need 221 arises. But one should always think about whether the need has truly 222 arisen and be prepared to explain why something abnormal needs to be done.
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