1.. _development_advancedtopics: 2 3Advanced topics 4=============== 5 6At this point, hopefully, you have a handle on how the development process 7works. There is still more to learn, however! This section will cover a 8number of topics which can be helpful for developers wanting to become a 9regular part of the Linux kernel development process. 10 11Managing patches with git 12------------------------- 13 14The use of distributed version control for the kernel began in early 2002, 15when Linus first started playing with the proprietary BitKeeper 16application. While BitKeeper was controversial, the approach to software 17version management it embodied most certainly was not. Distributed version 18control enabled an immediate acceleration of the kernel development 19project. In current times, there are several free alternatives to 20BitKeeper. For better or for worse, the kernel project has settled on git 21as its tool of choice. 22 23Managing patches with git can make life much easier for the developer, 24especially as the volume of those patches grows. Git also has its rough 25edges and poses certain hazards; it is a young and powerful tool which is 26still being civilized by its developers. This document will not attempt to 27teach the reader how to use git; that would be sufficient material for a 28long document in its own right. Instead, the focus here will be on how git 29fits into the kernel development process in particular. Developers who 30wish to come up to speed with git will find more information at: 31 32 https://git-scm.com/ 33 34 https://www.kernel.org/pub/software/scm/git/docs/user-manual.html 35 36and on various tutorials found on the web. 37 38The first order of business is to read the above sites and get a solid 39understanding of how git works before trying to use it to make patches 40available to others. A git-using developer should be able to obtain a copy 41of the mainline repository, explore the revision history, commit changes to 42the tree, use branches, etc. An understanding of git's tools for the 43rewriting of history (such as rebase) is also useful. Git comes with its 44own terminology and concepts; a new user of git should know about refs, 45remote branches, the index, fast-forward merges, pushes and pulls, detached 46heads, etc. It can all be a little intimidating at the outset, but the 47concepts are not that hard to grasp with a bit of study. 48 49Using git to generate patches for submission by email can be a good 50exercise while coming up to speed. 51 52When you are ready to start putting up git trees for others to look at, you 53will, of course, need a server that can be pulled from. Setting up such a 54server with git-daemon is relatively straightforward if you have a system 55which is accessible to the Internet. Otherwise, free, public hosting sites 56(Github, for example) are starting to appear on the net. Established 57developers can get an account on kernel.org, but those are not easy to come 58by; see https://kernel.org/faq/ for more information. 59 60The normal git workflow involves the use of a lot of branches. Each line 61of development can be separated into a separate "topic branch" and 62maintained independently. Branches in git are cheap, there is no reason to 63not make free use of them. And, in any case, you should not do your 64development in any branch which you intend to ask others to pull from. 65Publicly-available branches should be created with care; merge in patches 66from development branches when they are in complete form and ready to go - 67not before. 68 69Git provides some powerful tools which can allow you to rewrite your 70development history. An inconvenient patch (one which breaks bisection, 71say, or which has some other sort of obvious bug) can be fixed in place or 72made to disappear from the history entirely. A patch series can be 73rewritten as if it had been written on top of today's mainline, even though 74you have been working on it for months. Changes can be transparently 75shifted from one branch to another. And so on. Judicious use of git's 76ability to revise history can help in the creation of clean patch sets with 77fewer problems. 78 79Excessive use of this capability can lead to other problems, though, beyond 80a simple obsession for the creation of the perfect project history. 81Rewriting history will rewrite the changes contained in that history, 82turning a tested (hopefully) kernel tree into an untested one. But, beyond 83that, developers cannot easily collaborate if they do not have a shared 84view of the project history; if you rewrite history which other developers 85have pulled into their repositories, you will make life much more difficult 86for those developers. So a simple rule of thumb applies here: history 87which has been exported to others should generally be seen as immutable 88thereafter. 89 90So, once you push a set of changes to your publicly-available server, those 91changes should not be rewritten. Git will attempt to enforce this rule if 92you 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 94override this check, and there may be times when it is necessary to rewrite 95an exported tree. Moving changesets between trees to avoid conflicts in 96linux-next is one example. But such actions should be rare. This is one 97of the reasons why development should be done in private branches (which 98can be rewritten if necessary) and only moved into public branches when 99it's in a reasonably advanced state. 100 101As the mainline (or other tree upon which a set of changes is based) 102advances, it is tempting to merge with that tree to stay on the leading 103edge. For a private branch, rebasing can be an easy way to keep up with 104another tree, but rebasing is not an option once a tree is exported to the 105world. Once that happens, a full merge must be done. Merging occasionally 106makes good sense, but overly frequent merges can clutter the history 107needlessly. Suggested technique in this case is to merge infrequently, and 108generally only at specific release points (such as a mainline -rc 109release). If you are nervous about specific changes, you can always 110perform test merges in a private branch. The git "rerere" tool can be 111useful in such situations; it remembers how merge conflicts were resolved 112so that you don't have to do the same work twice. 113 114One of the biggest recurring complaints about tools like git is this: the 115mass movement of patches from one repository to another makes it easy to 116slip in ill-advised changes which go into the mainline below the review 117radar. Kernel developers tend to get unhappy when they see that kind of 118thing happening; putting up a git tree with unreviewed or off-topic patches 119can affect your ability to get trees pulled in the future. Quoting Linus: 120 121:: 122 123 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. 127 128(https://lwn.net/Articles/224135/). 129 130To avoid this kind of situation, ensure that all patches within a given 131branch stick closely to the associated topic; a "driver fixes" branch 132should not be making changes to the core memory management code. And, most 133importantly, do not use a git tree to bypass the review process. Post an 134occasional summary of the tree to the relevant list, and, when the time is 135right, request that the tree be included in linux-next. 136 137If and when others start to send patches for inclusion into your tree, 138don't forget to review them. Also ensure that you maintain the correct 139authorship information; the git "am" tool does its best in this regard, but 140you may have to add a "From:" line to the patch if it has been relayed to 141you via a third party. 142 143When requesting a pull, be sure to give all the relevant information: where 144your tree is, what branch to pull, and what changes will result from the 145pull. The git request-pull command can be helpful in this regard; it will 146format the request as other developers expect, and will also check to be 147sure that you have remembered to push those changes to the public server. 148 149 150Reviewing patches 151----------------- 152 153Some readers will certainly object to putting this section with "advanced 154topics" on the grounds that even beginning kernel developers should be 155reviewing patches. It is certainly true that there is no better way to 156learn how to program in the kernel environment than by looking at code 157posted by others. In addition, reviewers are forever in short supply; by 158looking at code you can make a significant contribution to the process as a 159whole. 160 161Reviewing code can be an intimidating prospect, especially for a new kernel 162developer who may well feel nervous about questioning code - in public - 163which has been posted by those with more experience. Even code written by 164the most experienced developers can be improved, though. Perhaps the best 165piece of advice for reviewers (all reviewers) is this: phrase review 166comments as questions rather than criticisms. Asking "how does the lock 167get released in this path?" will always work better than stating "the 168locking here is wrong." 169 170Different developers will review code from different points of view. Some 171are mostly concerned with coding style and whether code lines have trailing 172white space. Others will focus primarily on whether the change implemented 173by the patch as a whole is a good thing for the kernel or not. Yet others 174will check for problematic locking, excessive stack usage, possible 175security issues, duplication of code found elsewhere, adequate 176documentation, adverse effects on performance, user-space ABI changes, etc. 177All types of review, if they lead to better code going into the kernel, are 178welcome and worthwhile. 179