linux-kernel - Re: [Xen-devel] [PATCH RFC 03/12] x86/ticketlock: Use C for __ticket_spin

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Message-ID: <4C5C4AAB.3000606@zytor.com>
Date:	Fri, 06 Aug 2010 10:47:23 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Jeremy Fitzhardinge <jeremy@...p.org>
CC:	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Nick Piggin <npiggin@...e.de>,
	Peter Zijlstra <peterz@...radead.org>,
	Xen-devel <xen-devel@...ts.xensource.com>,
	Avi Kivity <avi@...hat.com>, Jan Beulich <JBeulich@...ell.com>
Subject: Re: [Xen-devel] [PATCH RFC 03/12] x86/ticketlock: Use C for __ticket_spin_unlock

On 07/20/2010 09:17 AM, Jeremy Fitzhardinge wrote:
> 
> "volatile" would be a compiler barrier, but it has no direct effect on,
> or relevence to, the CPU.  It just cares about the LOCK_PREFIX.  The
> "memory" clobber is probably unnecessary as well, since the constraints
> already tell the compiler the most important information.  We can add
> barriers separately as needed.
> 

You absolutely need volatile, since otherwise you're permitting the
compiler to split, re-execute or even drop the code.  Anything else
might work, by accident, but it's not clean.

	-hpa

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

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