linux-kernel - Re: [PATCH 2/2] ix86: atomic64 assembly improvements

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Date:	Wed, 18 Jan 2012 08:36:21 -0800
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Jan Beulich <JBeulich@...e.com>
CC:	mingo@...e.hu, tglx@...utronix.de,
	"eric.dumazet@...il.com" <eric.dumazet@...il.com>,
	luca@...a-barbieri.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] ix86: atomic64 assembly improvements

On 01/18/2012 06:24 AM, Jan Beulich wrote:
> The cmpxchg8b variants of "set" and "xchg" are really identical, and
> hence don't need to be repeated: %ebx and %ecx don't need to be copied
> into %eax and %edx respectively (this is only necessary when desiring
> to only read the stored value), and the LOCK prefix should also be used
> in "set" (other than the comment that is now being removed was saying,
> there is - to my knowledge - no *architectural* guarantee that aligned
> 64-bit writes would always be carried out atomically).

EWHAT?

It's atomic in the same way a MOV is atomic.

The CPU could, in fact, execute the locked version at all if the
unlocked version didn't behave like that.

Unless you have a specific instance where you think this might be
violated, please let me know.

	-hpa

-- 
H. Peter Anvin, Intel Open Source Technology Center
I work for Intel.  I don't speak on their behalf.

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