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

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Message-ID: <Pine.LNX.4.64.0804030035550.15850@artax.karlin.mff.cuni.cz>
Date:	Thu, 3 Apr 2008 00:39:54 +0200 (CEST)
From:	Mikulas Patocka <mikulas@...ax.karlin.mff.cuni.cz>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
cc:	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, Linus Torvalds wrote:

> On Wed, 2 Apr 2008, Linus Torvalds wrote:
> > 
> > Core 2 is the outlier in having a noticeably faster "mfence" than atomic 
> > instructions
> 
> Side note: mfence is probably faster than 6 cycles on Core 2. I've seen it 
> be basically zero-cost. Try adding a few memory ops around it - the timing 
> will probably change. Core 2 has this odd behaviour that it sometimes does 
> worse on the *really* trivial things that don't happen in real life.
> 
> 			Linus

When I added memory reads around mfence, the combined result was worse 
than the sum of reads and mfence --- mfence alone 8 ticks, mfence+4reads 
18 ticks.

mfence+writes scales linearly, i.e. mfence + 4 writes does 12 ticks.

mfence can fully overlap with register operations.

Mikulas
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