linux-kernel - Re: shmem_fill_super(): WARNING: kmemcheck: Caught 32-bit read from uninitialized memory

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Message-Id: <1253531239.5216.27.camel@penberg-laptop>
Date:	Mon, 21 Sep 2009 14:07:19 +0300
From:	Pekka Enberg <penberg@...helsinki.fi>
To:	Hugh Dickins <hugh.dickins@...cali.co.uk>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Vegard Nossum <vegard.nossum@...il.com>,
	Ingo Molnar <mingo@...e.hu>, linux-kernel@...r.kernel.org,
	Eric Paris <eparis@...hat.com>
Subject: Re: shmem_fill_super(): WARNING: kmemcheck: Caught 32-bit read
 from uninitialized memory

Hi Hugh,

On Mon, 2009-09-21 at 11:49 +0100, Hugh Dickins wrote:
> I got a bit anxious when I saw that the mode arg to shmem_get_inode()
> is declared as an int: was afraid that compiler was then passing a bad
> upper half down, which in fact would cause no trouble, but how could it
> be sure of that?  However, it looks okay: after doing the 32-bit load,
> it goes on to do a movzwl %ax,%eax - seems an odd way to proceed to me,
> but I bet it knows a lot more about efficiency of memory loads than I do.

Yeah, that's a common cause of kmemcheck false positives. I guess GCC
wants to avoid a partial register stall in the memory load and expects
CPU register renaming to work for "movzwl %ax, %eax" or something.

			Pekka

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