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Subject: Re: [PATCH v6 5/6] MCS Lock: Restructure the MCS lock defines and
 locking code into its own file
From: Jason Low <jason.low2@hp.com>
Date: Mon, 30 Sep 2013 09:10:40 -0700
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References: <cover.1380144003.git.tim.c.chen@linux.intel.com>
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To: Waiman Long <waiman.long@hp.com>
Cc: Paul McKenney <paulmck@linux.vnet.ibm.com>, Tim Chen <tim.c.chen@linux.intel.com>, Ingo Molnar <mingo@elte.hu>, Andrew Morton <akpm@linux-foundation.org>, Andrea Arcangeli <aarcange@redhat.com>, Alex Shi <alex.shi@linaro.org>, Andi Kleen <andi@firstfloor.org>, Michel Lespinasse <walken@google.com>, Davidlohr Bueso <davidlohr.bueso@hp.com>, Matthew R Wilcox <matthew.r.wilcox@intel.com>, Dave Hansen <dave.hansen@intel.com>, Peter Zijlstra <a.p.zijlstra@chello.nl>, Rik van Riel <riel@redhat.com>, Peter Hurley <peter@hurleysoftware.com>, linux-kernel@vger.kernel.org, linux-mm <linux-mm@kvack.org>

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.

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