Re: [PATCH v7 4/6] MCS Lock: Barrier corrections

[Paul E. McKenney]

On Thu, Jan 16, 2014 at 04:08:28PM -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.
> 
> Note that using the smp_load_acquire/smp_store_release pair is not
> sufficient to form a full memory barrier across
> cpus for many architectures (except x86) for mcs_unlock and mcs_lock.
> For applications that absolutely need a full barrier across multiple cpus
> with mcs_unlock and mcs_lock pair, smp_mb__after_unlock_lock() should be
> used after mcs_lock if a full memory barrier needs to be guaranteed.
> 
> From: Waiman Long <Waiman.Long@xxxxxx>
> 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>

And this fixes my gripes in the first patch in this series, good!

Reviewed-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>

> ---
>  kernel/locking/mcs_spinlock.c | 18 +++++++++++++-----
>  1 file changed, 13 insertions(+), 5 deletions(-)
> 
> diff --git a/kernel/locking/mcs_spinlock.c b/kernel/locking/mcs_spinlock.c
> index 44fb092..6cdc730 100644
> --- a/kernel/locking/mcs_spinlock.c
> +++ b/kernel/locking/mcs_spinlock.c
> @@ -43,9 +43,12 @@ void mcs_spin_lock(struct mcs_spinlock **lock, struct mcs_spinlock *node)
>  		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 +71,12 @@ 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);
>  }
>  EXPORT_SYMBOL_GPL(mcs_spin_unlock);
> -- 
> 1.7.11.7
> 
> 
> 

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