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

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Message-ID: <alpine.LFD.2.00.0912171440420.15740@localhost.localdomain>
Date:	Thu, 17 Dec 2009 14:43:04 -0800 (PST)
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Alain Knaff <alain@...ff.lu>
cc:	markh@...pro.net, fdutils@...tils.linux.lu,
	linux-kernel@...r.kernel.org
Subject: Re: DMA cache consistency bug introduced in 2.6.28 (Was: Re: [Fdutils]
 Cannot format floppies under kernel 2.6.*?)



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 
commit.

> > 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).

			Linus
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