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

[Tim Chen]

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



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