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@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>
> ---
> 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
>
>
>
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