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

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

On Fri, Nov 08, 2013 at 11:52:38AM -0800, Tim Chen wrote:
> From: Waiman Long <Waiman.Long@hp.com>
> 
> 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@google.com>
> Signed-off-by: Waiman Long <Waiman.Long@hp.com>
> Signed-off-by: Jason Low <jason.low2@hp.com>
> Signed-off-by: Tim Chen <tim.c.chen@linux.intel.com>

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.

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?

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