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Re: [Qemu-devel] Re: [PATCH] block: fix sector comparism in multiwrite_r

From: Kevin Wolf
Subject: Re: [Qemu-devel] Re: [PATCH] block: fix sector comparism in multiwrite_req_compare
Date: Wed, 19 May 2010 23:09:07 +0200
User-agent: KMail/1.9.5

Am Mittwoch, 19. Mai 2010 21:42 schrieb Michael Tokarev:
> 19.05.2010 23:38, Christoph Hellwig wrote:
> > On Wed, May 19, 2010 at 11:26:58PM +0400, Michael Tokarev wrote:
> >> Now the question is if the previous patch by Avi is actually
> >> worth to apply -- I mean this one:
> >>   [Qemu-devel] [PATCH +stable] block: don't attempt to merge overlapping
> >>   requests
> >
> > No, that was just a hack, it shouldn't be applied.
> Actually it's not that obvious.  If the actual problem
> here (besides the mis-comparison) is due to missing
> barriers or flushes.  Avi asked a good question in that
> thread.

It's obvious that it's a hack. It doesn't fix anything, it just disables a 
feature that didn't work. Good for debugging, but not something that you 
would like to commit.

It's reasonable to include something like this when we know that something is 
broken but we haven't found it yet - but I believe Christoph's patch is the 
real fix. If anyone can still find a case that is "fixed" by Avi's patch, I 
could be convinced to apply it anyway, but I'd prefer if I didn't have to.

Note that we actually don't have overlapping requests. It just looks like it 
because the qsort call doesn't work correctly with the broken comparison 
function, so lower sector numbers can come after higher ones.


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