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

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Date:	Wed, 2 Apr 2008 15:51:56 -0700 (PDT)
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Mikulas Patocka <mikulas@...ax.karlin.mff.cuni.cz>
cc:	viro@...iv.linux.org.uk, linux-kernel@...r.kernel.org
Subject: Re: [PATCH]: Fix SMP-reordering race in mark_buffer_dirty



On Thu, 3 Apr 2008, Mikulas Patocka wrote:
> 
> 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.

Sorry, my test-program was buggy.

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

I get different results now that I fixed my memop test. I get a loop with 
reads and writes taking 3 cycles without mfence, and 21 cycles with (read 
after, write before). 

			Linus
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