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.