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