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> --- 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 -- To unsubscribe from this list: send the line "unsubscribe linux-arch" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
