linux-kernel - Re: [PATCH] slub: fix check

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Date:	Mon, 8 Aug 2011 23:24:18 +0200
From:	Marcin Slusarz <marcin.slusarz@...il.com>
To:	Akinobu Mita <akinobu.mita@...il.com>
Cc:	linux-kernel@...r.kernel.org,
	Christoph Lameter <cl@...ux-foundation.org>,
	Pekka Enberg <penberg@...nel.org>,
	Matt Mackall <mpm@...enic.com>, linux-mm@...ck.org
Subject: Re: [PATCH] slub: fix check_bytes() for slub debugging

On Sun, Aug 07, 2011 at 06:30:38PM +0900, Akinobu Mita wrote:
> The check_bytes() function is used by slub debugging.  It returns a pointer
> to the first unmatching byte for a character in the given memory area.
> 
> If the character for matching byte is greater than 0x80, check_bytes()
> doesn't work.  Becuase 64-bit pattern is generated as below.
> 
> 	value64 = value | value << 8 | value << 16 | value << 24;
> 	value64 = value64 | value64 << 32;
> 
> The integer promotions are performed and sign-extended as the type of value
> is u8.  The upper 32 bits of value64 is 0xffffffff in the first line, and
> the second line has no effect.
> 
> This fixes the 64-bit pattern generation.

Thank you. I'm a bit ashamed about this bug... I introduced this bug, so:
Reviewed-by: Marcin Slusarz <marcin.slusarz@...il.com>

I tested your patch to check if performance improvements of commit
c4089f98e943ff445665dea49c190657b34ccffe come from this bug or not.
And forunately they aren't - performance is exactly the same.

How did you find it?

Marcin
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