linux-kernel - Re: [PATCH v6 5/6] MCS Lock: Restructure the MCS lock defines and locking code into its own file

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Date:	Tue, 01 Oct 2013 09:48:12 -0700
From:	Tim Chen <tim.c.chen@...ux.intel.com>
To:	Waiman Long <waiman.long@...com>
Cc:	Jason Low <jason.low2@...com>,
	Paul McKenney <paulmck@...ux.vnet.ibm.com>,
	Ingo Molnar <mingo@...e.hu>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Alex Shi <alex.shi@...aro.org>,
	Andi Kleen <andi@...stfloor.org>,
	Michel Lespinasse <walken@...gle.com>,
	Davidlohr Bueso <davidlohr.bueso@...com>,
	Matthew R Wilcox <matthew.r.wilcox@...el.com>,
	Dave Hansen <dave.hansen@...el.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Rik van Riel <riel@...hat.com>,
	Peter Hurley <peter@...leysoftware.com>,
	linux-kernel@...r.kernel.org, linux-mm <linux-mm@...ck.org>
Subject: Re: [PATCH v6 5/6] MCS Lock: Restructure the MCS lock defines and
 locking code into its own file

On Mon, 2013-09-30 at 12:36 -0400, Waiman Long wrote:
> On 09/30/2013 12:10 PM, Jason Low wrote:
> > On Mon, 2013-09-30 at 11:51 -0400, Waiman Long wrote:
> >> On 09/28/2013 12:34 AM, Jason Low wrote:
> >>>> Also, below is what the mcs_spin_lock() and mcs_spin_unlock()
> >>>> functions would look like after applying the proposed changes.
> >>>>
> >>>> static noinline
> >>>> void mcs_spin_lock(struct mcs_spin_node **lock, struct mcs_spin_node *node)
> >>>> {
> >>>>           struct mcs_spin_node *prev;
> >>>>
> >>>>           /* Init node */
> >>>>           node->locked = 0;
> >>>>           node->next   = NULL;
> >>>>
> >>>>           prev = xchg(lock, node);
> >>>>           if (likely(prev == NULL)) {
> >>>>                   /* Lock acquired. No need to set node->locked since it
> >>>> won't be used */
> >>>>                   return;
> >>>>           }
> >>>>           ACCESS_ONCE(prev->next) = node;
> >>>>           /* Wait until the lock holder passes the lock down */
> >>>>           while (!ACCESS_ONCE(node->locked))
> >>>>                   arch_mutex_cpu_relax();
> >>>>           smp_mb();
> >> I wonder if a memory barrier is really needed here.
> > If the compiler can reorder the while (!ACCESS_ONCE(node->locked)) check
> > so that the check occurs after an instruction in the critical section,
> > then the barrier may be necessary.
> >
> 
> In that case, just a barrier() call should be enough.

The cpu could still be executing out of order load instruction from the
critical section before checking node->locked?  Probably smp_mb() is
still needed.

Tim


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