linux-kernel - Re: [PATCH 06/20] x86/ticketlock: make __ticket_spin

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Message-ID: <4CE18C57.9070805@goop.org>
Date:	Mon, 15 Nov 2010 11:39:03 -0800
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	Eric Dumazet <eric.dumazet@...il.com>
CC:	Américo Wang <xiyou.wangcong@...il.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Nick Piggin <npiggin@...e.de>,
	Jan Beulich <JBeulich@...ell.com>, Avi Kivity <avi@...hat.com>,
	Xen-devel <xen-devel@...ts.xensource.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	Linux Virtualization <virtualization@...ts.linux-foundation.org>,
	Srivatsa Vaddagiri <vatsa@...ux.vnet.ibm.com>,
	Jeremy Fitzhardinge <jeremy.fitzhardinge@...rix.com>
Subject: Re: [PATCH 06/20] x86/ticketlock: make __ticket_spin_trylock common

On 11/13/2010 02:48 AM, Eric Dumazet wrote:
> Le samedi 13 novembre 2010 à 18:17 +0800, Américo Wang a écrit :
>> On Wed, Nov 03, 2010 at 10:59:47AM -0400, Jeremy Fitzhardinge wrote:
>>
>>> +	union {
>>> +		struct __raw_tickets tickets;
>>> +		__ticketpair_t slock;
>>> +	} tmp, new;
>>> +	int ret;
>>> +
>>> +	tmp.tickets = ACCESS_ONCE(lock->tickets);
>>> +	if (tmp.tickets.head != tmp.tickets.tail)
>>> +		return 0;
>>> +
>>> +	new.slock = tmp.slock + (1 << TICKET_SHIFT);
>>> +
>>> +	ret = cmpxchg(&lock->ticketpair, tmp.slock, new.slock) == tmp.slock;
>>> +	barrier();		/* just make nothing creeps before lock is claimed */
>> This one should be smp_wmb(), right? No CONFIG_X86_OOSTORE protected.
> cmpxchg() is a full memory barrier, no need for smp_wmb() or barrier()

Agreed.

    J
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