netdev - Re: Seeing new kernel unaligned access messages in linux-next on ia64

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Message-ID: <m2vdcl5ocj.fsf@igel.home>
Date:	Wed, 24 Mar 2010 23:47:24 +0100
From:	Andreas Schwab <schwab@...ux-m68k.org>
To:	Jan Engelhardt <jengelh@...ozas.de>
Cc:	David Miller <davem@...emloft.net>, tony.luck@...el.com,
	netdev@...r.kernel.org
Subject: Re: Seeing new kernel unaligned access messages in linux-next on ia64

Jan Engelhardt <jengelh@...ozas.de> writes:

> Using a temporary and memcpying it off would normally fix this,
> as in the patch below. During testing however, I still get
> unaligned messages even with the patch - and I would not know
> what causes this.

The memcpy will not fix the alignment issue because the copy operation
is fully equivalent to a direct assignment, and the compiler can still
take advantage of the known alignment of the types.  You have to
explicitly tell the compiler about the reduced alignment guarantee.

> In fact, adding a printks magically fixes it. (Bug in gcc-4.4-sparc
> compiler?)
>
>  	memcpy(v, &a, sizeof(a));
> +       printk(KERN_INFO "v=%p a=%p\n", v, &a);

Presumably the extended lifetime of the variables caused the compiler to
use a different expansion for memcpy which is less dependent on
alignment.

Andreas.

-- 
Andreas Schwab, schwab@...ux-m68k.org
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