linux-kernel - Re: [patch 134/149] x86, paravirt: Add a global synchronization point for pvclock

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Date:	Tue, 13 Jul 2010 11:21:03 -0700
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
CC:	Peter Palfrader <peter@...frader.org>, Avi Kivity <avi@...hat.com>,
	Greg KH <gregkh@...e.de>, linux-kernel@...r.kernel.org,
	stable@...nel.org, stable-review@...nel.org,
	akpm@...ux-foundation.org, alan@...rguk.ukuu.org.uk,
	Glauber Costa <glommer@...hat.com>,
	Zachary Amsden <zamsden@...hat.com>,
	Marcelo Tosatti <mtosatti@...hat.com>
Subject: Re: [patch 134/149] x86, paravirt: Add a global synchronization point
 	for pvclock

On 07/13/2010 10:59 AM, Linus Torvalds wrote:
> Btw, this second patch was a bit more aggressive than the first one,
> and actually removes the "memory" clobber entirely, and the fake cast
> of the target type.
>
> That shouldn't matter _except_ if people actually use cmpxchg to
> implement their own locking, since now the compiler could potentially
> move unrelated memory references around the lock. Of course, if you
> use cmpxchg to implement your own locking, you're probably doing
> something wrong anyway (ie you'll get the wrong memory barriers on
> various architectures), so it should all be fine.
>   

There are some places which rely on xchg/cmpxchg being a barrier in
arch-specific code.  For example, the Xen code uses as part of the
shared memory protocol with the hypervisor.

> But I thought I'd mention it. And I don't really know how much gcc
> moves memory accesses around a "asm volatile" - the gcc docs are
> historically very vague ("volatile asms aren't moved around
> 'significantly'", whatever 'significant' means)
>   

"asm volatile"'s only real meaning is that it will not get elided if it
appears its output is unused (assuming it is reachable at all).  I don't
think you can consider it having any meaningful effects on ordering.

    J
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