Re: [PATCH v5 4/4] MCS Lock: Barrier corrections

[Tim Chen]

On Tue, 2013-11-19 at 11:21 -0800, Paul E. McKenney wrote:
> On Fri, Nov 08, 2013 at 11:52:38AM -0800, Tim Chen wrote:
> > From: Waiman Long <Waiman.Long@xxxxxx>
> > 
> > This patch corrects the way memory barriers are used in the MCS lock
> > with smp_load_acquire and smp_store_release fucnction.
> > It removes ones that are not needed.
> > 
> > It uses architecture specific load-acquire and store-release
> > primitives for synchronization, if available. Generic implementations
> > are provided in case they are not defined even though they may not
> > be optimal. These generic implementation could be removed later on
> > once changes are made in all the relevant header files.
> > 
> > Suggested-by: Michel Lespinasse <walken@xxxxxxxxxx>
> > Signed-off-by: Waiman Long <Waiman.Long@xxxxxx>
> > Signed-off-by: Jason Low <jason.low2@xxxxxx>
> > Signed-off-by: Tim Chen <tim.c.chen@xxxxxxxxxxxxxxx>
> 
> Please see comments below.
> 
> 							Thanx, Paul
> 
> > ---
> >  kernel/locking/mcs_spinlock.c |   48 +++++++++++++++++++++++++++++++++++------
> >  1 files changed, 41 insertions(+), 7 deletions(-)
> > 
> > diff --git a/kernel/locking/mcs_spinlock.c b/kernel/locking/mcs_spinlock.c
> > index b6f27f8..df5c167 100644
> > --- a/kernel/locking/mcs_spinlock.c
> > +++ b/kernel/locking/mcs_spinlock.c
> > @@ -23,6 +23,31 @@
> >  #endif
> > 
> >  /*
> > + * Fall back to use the regular atomic operations and memory barrier if
> > + * the acquire/release versions are not defined.
> > + */
> > +#ifndef	xchg_acquire
> > +# define xchg_acquire(p, v)		xchg(p, v)
> > +#endif
> > +
> > +#ifndef	smp_load_acquire
> > +# define smp_load_acquire(p)				\
> > +	({						\
> > +		typeof(*p) __v = ACCESS_ONCE(*(p));	\
> > +		smp_mb();				\
> > +		__v;					\
> > +	})
> > +#endif
> > +
> > +#ifndef smp_store_release
> > +# define smp_store_release(p, v)		\
> > +	do {					\
> > +		smp_mb();			\
> > +		ACCESS_ONCE(*(p)) = v;		\
> > +	} while (0)
> > +#endif
> > +
> > +/*
> >   * 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
> > @@ -37,15 +62,19 @@ void mcs_spin_lock(struct mcs_spinlock **lock, struct mcs_spinlock *node)
> >  	node->locked = 0;
> >  	node->next   = NULL;
> > 
> > -	prev = xchg(lock, node);
> > +	/* xchg() provides a memory barrier */
> > +	prev = xchg_acquire(lock, node);
> 
> But if this is xchg_acquire() with only acquire semantics, it need not
> ensure that the initializations of node->locked and node->next above
> will happen before the "ACCESS_ONCE(prev->next) = node" below.  This
> therefore needs to remain xchg().  Or you need an smp_store_release()
> below instead of an ACCESS_ONCE() assignment.

Good point.  Will keep it as xchg.

> 
> As currently written, the poor CPU doing the unlock can be fatally
> disappointed by seeing pre-initialized values of ->locked and ->next.
> This could, among other things, result in a hang where the handoff
> happens before the initialization.
> 
> >  	if (likely(prev == NULL)) {
> >  		/* Lock acquired */
> >  		return;
> >  	}
> >  	ACCESS_ONCE(prev->next) = node;
> > -	smp_wmb();
> > -	/* Wait until the lock holder passes the lock down */
> > -	while (!ACCESS_ONCE(node->locked))
> > +	/*
> > +	 * Wait until the lock holder passes the lock down.
> > +	 * Using smp_load_acquire() provides a memory barrier that
> > +	 * ensures subsequent operations happen after the lock is acquired.
> > +	 */
> > +	while (!(smp_load_acquire(&node->locked)))
> >  		arch_mutex_cpu_relax();
> 
> OK, this smp_load_acquire() makes sense!
> 
> >  }
> >  EXPORT_SYMBOL_GPL(mcs_spin_lock);
> > @@ -54,7 +83,7 @@ EXPORT_SYMBOL_GPL(mcs_spin_lock);
> >   * 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)
> > +void mcs_spin_unlock(struct mcs_spinlock **lock, struct mcs_spinlock *node)
> >  {
> >  	struct mcs_spinlock *next = ACCESS_ONCE(node->next);
> > 
> > @@ -68,7 +97,12 @@ static void mcs_spin_unlock(struct mcs_spinlock **lock, struct mcs_spinlock *nod
> >  		while (!(next = ACCESS_ONCE(node->next)))
> >  			arch_mutex_cpu_relax();
> >  	}
> > -	ACCESS_ONCE(next->locked) = 1;
> > -	smp_wmb();
> > +	/*
> > +	 * Pass lock to next waiter.
> > +	 * smp_store_release() provides a memory barrier to ensure
> > +	 * all operations in the critical section has been completed
> > +	 * before unlocking.
> > +	 */
> > +	smp_store_release(&next->locked , 1);
> 
> This smp_store_release() makes sense as well!
> 
> Could you please get rid of the extraneous space before the comma?

Will do.

> 
> >  }
> >  EXPORT_SYMBOL_GPL(mcs_spin_unlock);
> > -- 
> > 1.7.4.4
> > 
> > 
> 


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