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Date: Thu, 20 Nov 2008 22:03:37 +0200
From: Pekka Enberg <penberg@cs.helsinki.fi>
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To: Valdis.Kletnieks@vt.edu
CC: Cyrill Gorcunov <gorcunov@gmail.com>,
       Christoph Lameter <cl@linux-foundation.org>,
       LKML <linux-kernel@vger.kernel.org>,
       Andrew Morton <akpm@linux-foundation.org>
Subject: Re: mm: slab - __cache_alloc NULL prefetch fix
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Valdis.Kletnieks@vt.edu wrote:
> On Thu, 20 Nov 2008 19:44:00 +0300, Cyrill Gorcunov said:
> 
>> -	prefetchw(objp);
>>  
>> -	if (likely(objp))
>> +	if (likely(objp)) {
>> +		prefetchw(objp);
>>  		kmemcheck_slab_alloc(cachep, flags, objp, obj_size(cachep));
>> +	}
> 
> Although it probably makes sense to not bother prefetching NULL, I also
> need to wonder how useful it is to prefetch something that we then
> turn around and dereference in the very next line of code.

Note that kmemcheck_slab_alloc() is going to be a no-op on anything but 
a special developer or tester debug config.