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

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Message-ID: <4F182FCB.2090300@zytor.com>
Date:	Thu, 19 Jan 2012 06:59:23 -0800
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Jan Beulich <JBeulich@...e.com>
CC:	mingo@...e.hu, "eric.dumazet@...il.com" <eric.dumazet@...il.com>,
	tglx@...utronix.de, luca@...a-barbieri.com,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] ix86: atomic64 assembly improvements

On 01/19/2012 06:50 AM, Jan Beulich wrote:
>>
>> It doesn't have anything to do with caches on or off.
>
> How does it not? If any part of the bus topology is only 32 bits wide,
> a 64-bit read or write simply can't be executed atomically without
> asserting LOCK#.
>

Furthermore, if you're going down that rathole then we can't even trust 
MOV (nor, for that matter, can you trust LOCK in a lot of 
circumstances.)  In short, you need to be extremely careful about what 
you do to uncached memory *at all* (and you need to know exactly what is 
behind this bus) but pessimizing these kinds of construct for that 
reason is wrong in the extreme.

	-hpa

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