Re: [PATCH v5 2/4] MCS Lock: optimizations and extra comments

[Tim Chen <tim.c.chen@linux.intel.com>]

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:
> > From: Jason Low <jason.low2@hp.com>
> > 
> > Remove unnecessary operation and make the cmpxchg(lock, node, NULL) == node
> > check in mcs_spin_unlock() likely() as it is likely that a race did not occur
> > most of the time.
> > 
> > Also add in more comments describing how the local node is used in MCS locks.
> > 
> > Reviewed-by: Tim Chen <tim.c.chen@linux.intel.com>
> > Signed-off-by: Jason Low <jason.low2@hp.com>
> > ---
> >  include/linux/mcs_spinlock.h |   13 +++++++++++--
> >  1 files changed, 11 insertions(+), 2 deletions(-)
> > 
> > diff --git a/include/linux/mcs_spinlock.h b/include/linux/mcs_spinlock.h
> > index b5de3b0..96f14299 100644
> > --- a/include/linux/mcs_spinlock.h
> > +++ b/include/linux/mcs_spinlock.h
> > @@ -18,6 +18,12 @@ struct mcs_spinlock {
> >  };
> > 
> >  /*
> > + * In order to acquire the lock, the caller should declare a local node and
> > + * pass a reference of the node to this function in addition to the lock.
> > + * If the lock has already been acquired, then this will proceed to spin
> > + * on this node->locked until the previous lock holder sets the node->locked
> > + * in mcs_spin_unlock().
> > + *
> >   * We don't inline mcs_spin_lock() so that perf can correctly account for the
> >   * time spent in this lock function.
> >   */
> > @@ -33,7 +39,6 @@ void mcs_spin_lock(struct mcs_spinlock **lock, struct mcs_spinlock *node)
> >  	prev = xchg(lock, node);
> >  	if (likely(prev == NULL)) {
> >  		/* Lock acquired */
> > -		node->locked = 1;
> 
> Agreed, no one looks at this field in this case, so no need to initialize
> it, unless for debug purposes.
> 
> >  		return;
> >  	}
> >  	ACCESS_ONCE(prev->next) = node;
> > @@ -43,6 +48,10 @@ void mcs_spin_lock(struct mcs_spinlock **lock, struct mcs_spinlock *node)
> >  		arch_mutex_cpu_relax();
> >  }
> > 
> > +/*
> > + * 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", and
> if the issues that I called out in patch #1 are resolved:

I haven't instrumented the code to check the hit rate of this path. But
the slow path probably will only get hit in some cases under 
heavy contention. 


> 
> Reviewed-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
> 
> >  			return;
> >  		/* Wait until the next pointer is set */
> >  		while (!(next = ACCESS_ONCE(node->next)))
> > -- 
> > 1.7.4.4
> > 
> > 
> > 
> 
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