linux-kernel - Re: [PATCH]: Fix SMP-reordering race in mark_buffer

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Message-ID: <alpine.LFD.1.00.0804021505160.14670@woody.linux-foundation.org>
Date:	Wed, 2 Apr 2008 15:07:22 -0700 (PDT)
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Andrew Morton <akpm@...ux-foundation.org>
cc:	mikulas@...ax.karlin.mff.cuni.cz, viro@...iv.linux.org.uk,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH]: Fix SMP-reordering race in mark_buffer_dirty



On Wed, 2 Apr 2008, Andrew Morton wrote:
> 
> But then the test-and-set of an already-set flag would newly cause the
> cacheline to be dirtied, requiring additional bus usage to write it back?
> 
> The CPU's test-and-set-bit operation could of course optimise that away in
> this case.  But does it?

No, afaik no current x86 uarch will optimize away the write on a locked 
instuction if it turns out to be unnecessary. 

Can somebody find a timing reason to have the ugly code?

		Linus
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