[PATCH 3/4] MCS Lock: Barrier corrections

[Tim Chen <tim.c.chen@linux.intel.com>]

This patch corrects the way memory barriers are used in the MCS lock
and removes ones that are not needed. Also add comments on all barriers.

Reviewed-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> 
Reviewed-by: Tim Chen <tim.c.chen@linux.intel.com>
Signed-off-by: Jason Low <jason.low2@hp.com>
---
 include/linux/mcs_spinlock.h |   13 +++++++++++--
 1 files changed, 11 insertions(+), 2 deletions(-)

diff --git a/include/linux/mcs_spinlock.h b/include/linux/mcs_spinlock.h
index 96f14299..93d445d 100644
--- a/include/linux/mcs_spinlock.h
+++ b/include/linux/mcs_spinlock.h
@@ -36,16 +36,19 @@ 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))
 		arch_mutex_cpu_relax();
+
+	/* Make sure subsequent operations happen after the lock is acquired */
+	smp_rmb();
 }
 
 /*
@@ -58,6 +61,7 @@ static void mcs_spin_unlock(struct mcs_spinlock **lock, struct mcs_spinlock *nod
 
 	if (likely(!next)) {
 		/*
+		 * cmpxchg() provides a memory barrier.
 		 * Release the lock by setting it to NULL
 		 */
 		if (likely(cmpxchg(lock, node, NULL) == node))
@@ -65,9 +69,14 @@ static void mcs_spin_unlock(struct mcs_spinlock **lock, struct mcs_spinlock *nod
 		/* Wait until the next pointer is set */
 		while (!(next = ACCESS_ONCE(node->next)))
 			arch_mutex_cpu_relax();
+	} else {
+		/*
+		 * Make sure all operations within the critical section
+		 * happen before the lock is released.
+		 */
+		smp_wmb();
 	}
 	ACCESS_ONCE(next->locked) = 1;
-	smp_wmb();
 }
 
 #endif /* __LINUX_MCS_SPINLOCK_H */
-- 
1.7.4.4



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