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

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Date:	Sat, 13 Nov 2010 11:48:41 +0100
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Américo Wang <xiyou.wangcong@...il.com>
Cc:	Jeremy Fitzhardinge <jeremy@...p.org>,
	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

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()



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