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

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Date:	Thu, 19 Jan 2012 15:11:54 +0000
From:	"Jan Beulich" <JBeulich@...e.com>
To:	"H. Peter Anvin" <hpa@...or.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 19.01.12 at 15:59, "H. Peter Anvin" <hpa@...or.com> wrote:
> 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 

No - (general register) MOV is at most 32 bits wide.

> 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.

Okay, despite not being fully convinced I'll undo the folding of set and
xchg then, and re-submit.

Jan

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