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: <4B2ABDC8.6090104@knaff.lu>
Date:	Fri, 18 Dec 2009 00:24:56 +0100
From:	Alain Knaff <alain@...ff.lu>
To:	Linus Torvalds <torvalds@...ux-foundation.org>, markh@...pro.net
CC:	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.*?)

Linus Torvalds wrote:
> 
> 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

thanks for these explanations, that makes it clearer indeed.

Now, I only need to find a machine locally to test this on. Or Mark: are
you confident in doing this yourself?

Thanks,

Alain

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