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

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Date:	Wed, 18 Jan 2012 16:50:49 +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 18.01.12 at 17:36, "H. Peter Anvin" <hpa@...or.com> wrote:
> 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.

Then please point me to where this is documented.

As I understand it, there is nothing keeping the CPU (or something
down the bus) from executing the unlocked version as two 32-bit
reads followed by two 32-bit writes.

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

Assuming you meant "could not", that's not true: As long as the
external world has a way to know that both items are locked (think
of the old bus lock mechanism when there were no caches yet),
it can very well do so.

I do not question that in practice all CPUs behave as described,
but without an architectural guarantee (and with the code in
question not being used in hot paths afaik) I see no reason why
it should depend on undefined behavior.

Jan

> 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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