Re: DMA cache consistency bug introduced in 2.6.28 (Was: Re: [Fdutils]Cannot format floppies under kernel 2.6.*?)

From: Linus Torvalds
Date: Thu Dec 17 2009 - 17:43:33 EST

On Thu, 17 Dec 2009, Alain Knaff wrote:
> [...]
> > You'd need a git tree that contains both the working and non-working
> > versions, and then literally just do
> >
> > git bisect start
> > git bisect good <known good version number here>
> > git bisect bad <known bad version here>
> >
> > and it will give you a commit to try. Compile, test, see if it's good or
> > bad, and do
> >
> > git bisect [good|bad]
> >
> > depending on the result. Rinse and repeat (depending on how tight the
> > initial good/bad commits were, it will need 10-15 kernel tests).
> ... and how do I check out the most recent good / oldest bad kernel for
> compilation?

'git bisect' does all that for you. You don't need to check out the
kernels you mark good or bad - git will just calculate the commit graphs,
and pick a commit that is in the "middle" between them, and check out that

> > So after a successful bisect, it is usually a good idea to try to go back
> > to the original known-bad kernel, and then revert the commit that was
> > indicated as the bad one (assuming the revert works - it could be that the
> > bad one ends up being fundamental to other commits after it), and test
> > that yes, that really fixes the bug.
> What command lines would I use for that revert?

git revert <sha1-that-git-bisect-reported>

but even if that revert isn't successful, just the bisection result will
be very interesting (assuming it all looks sane, of course - as mentioned,
sometimes bisect results get screwed up because the bug isn't entirely
reproducible due to timing etc).

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