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Subject: Re: [PATCH v5 2/4] MCS Lock: optimizations and extra comments
From: Tim Chen <tim.c.chen@linux.intel.com>
To: paulmck@linux.vnet.ibm.com
Cc: Ingo Molnar <mingo@elte.hu>, Andrew Morton <akpm@linux-foundation.org>,
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Date: Tue, 19 Nov 2013 14:57:41 -0800
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On Tue, 2013-11-19 at 11:13 -0800, Paul E. McKenney wrote:
> On Fri, Nov 08, 2013 at 11:52:05AM -0800, Tim Chen wrote:
> > 
> > +/*
> > + * Releases the lock. The caller should pass in the corresponding node that
> > + * was used to acquire the lock.
> > + */
> >  static void mcs_spin_unlock(struct mcs_spinlock **lock, struct mcs_spinlock *node)
> >  {
> >  	struct mcs_spinlock *next = ACCESS_ONCE(node->next);
> > @@ -51,7 +60,7 @@ static void mcs_spin_unlock(struct mcs_spinlock **lock, struct mcs_spinlock *nod
> >  		/*
> >  		 * Release the lock by setting it to NULL
> >  		 */
> > -		if (cmpxchg(lock, node, NULL) == node)
> > +		if (likely(cmpxchg(lock, node, NULL) == node))
> 
> Agreed here as well.  Takes a narrow race to hit this.
> 
> So, did your testing exercise this path?  If the answer is "yes", 


Paul,

I did some instrumentation and confirmed that the path in question has 
been exercised.  So this patch should be okay.

Tim

> and if the issues that I called out in patch #1 are resolved:
> 
> Reviewed-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
> 
> >  			return;
> >  		/* Wait until the next pointer is set */
> >  		while (!(next = ACCESS_ONCE(node->next)))
> >