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diff --git a/howto/isolate-bugs-with-bisect.txt b/howto/isolate-bugs-with-bisect.txt new file mode 100644 index 0000000..4009495 --- /dev/null +++ b/howto/isolate-bugs-with-bisect.txt
@@ -0,0 +1,65 @@ +From: Linus Torvalds <torvalds () osdl ! org> +To: git@vger.kernel.org +Date: 2005-11-08 1:31:34 +Subject: Real-life kernel debugging scenario +Abstract: Short-n-sweet, Linus tells us how to leverage `git-bisect` to perform + bug isolation on a repository where "good" and "bad" revisions are known + in order to identify a suspect commit. + + +How To Use git-bisect To Isolate a Bogus Commit +=============================================== + +The way to use "git bisect" couldn't be easier. + +Figure out what the oldest bad state you know about is (that's usually the +head of "master", since that's what you just tried to boot and failed at). +Also, figure out the most recent known-good commit (usually the _previous_ +kernel you ran: and if you've only done a single "pull" in between, it +will be ORIG_HEAD). + +Then do + + git bisect start + git bisect bad master <- mark "master" as the bad state + git bisect good ORIG_HEAD <- mark ORIG_HEAD as good (or + whatever other known-good + thing you booted laste) + +and at this point "git bisect" will churn for a while, and tell you what +the mid-point between those two commits are, and check that state out as +the head of the bew "bisect" branch. + +Compile and reboot. + +If it's good, just do + + git bisect good <- mark current head as good + +otherwise, reboot into a good kernel instead, and do (surprise surprise, +git really is very intuitive): + + git bisect bad <- mark current head as bad + +and whatever you do, git will select a new half-way point. Do this for a +while, until git tells you exactly which commit was the first bad commit. +That's your culprit. + +It really works wonderfully well, except for the case where there was +_another_ commit that broke something in between, like introduced some +stupid compile error. In that case you should not mark that commit good or +bad: you should try to find another commit close-by, and do a "git reset +--hard <newcommit>" to try out _that_ commit instead, and then test that +instead (and mark it good or bad). + +You can do "git bisect visualize" while you do all this to see what's +going on by starting up gitk on the bisection range. + +Finally, once you've figured out exactly which commit was bad, you can +then go back to the master branch, and try reverting just that commit: + + git checkout master + git revert <bad-commit-id> + +to verify that the top-of-kernel works with that single commit reverted. +
