Re: [PATCH v6 4/5] MCS Lock: Barrier corrections

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

On Tue, Nov 19, 2013 at 05:37:43PM -0800, Tim Chen wrote:
> 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>
> ---
>  kernel/locking/mcs_spinlock.c | 19 ++++++++++++++-----
>  1 file changed, 14 insertions(+), 5 deletions(-)
> 
> diff --git a/kernel/locking/mcs_spinlock.c b/kernel/locking/mcs_spinlock.c
> index 44fb092..6f2ce8e 100644
> --- a/kernel/locking/mcs_spinlock.c
> +++ b/kernel/locking/mcs_spinlock.c
> @@ -37,15 +37,19 @@ void mcs_spin_lock(struct mcs_spinlock **lock, struct mcs_spinlock *node)
>  	node->locked = 0;
>  	node->next   = NULL;
> 
> +	/* xchg() provides a memory barrier */
>  	prev = xchg(lock, node);
>  	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();
>  }
>  EXPORT_SYMBOL_GPL(mcs_spin_lock);
> @@ -68,7 +72,12 @@ void mcs_spin_unlock(struct mcs_spinlock **lock, struct mcs_spinlock *node)
>  		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);

However, there is one problem with this that I missed yesterday.

Documentation/memory-barriers.txt requires that an unlock-lock pair
provide a full barrier, but this is not guaranteed if we use
smp_store_release() for unlock and smp_load_acquire() for lock.
At least one of these needs a full memory barrier.

							Thanx, Paul

>  }
>  EXPORT_SYMBOL_GPL(mcs_spin_unlock);
> -- 
> 1.7.11.7
> 
> 
>