From: Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>
Date: Sun, 20 Sep 2009 16:03:37 +0000 (-0400)
Subject: urcu-defer: remove unnecessary memory barrier
X-Git-Tag: v0.1~60
X-Git-Url: https://git.lttng.org./?a=commitdiff_plain;h=9374f873bd741e91b0b5c78a8e0406d849ca752e;p=userspace-rcu.git

urcu-defer: remove unnecessary memory barrier

All synchronization between queue producer/consumer is performed by the write to
"head". Before this write, none of the queued data is visible from the consumer
point of view.

Therefore, just a single wmb() is required before writing to head to ensure
correct synchronization. This matches with the rmb() after reading head on the
consumer side.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>
---

diff --git a/urcu-defer-static.h b/urcu-defer-static.h
index 4cfa837..a00ced0 100644
--- a/urcu-defer-static.h
+++ b/urcu-defer-static.h
@@ -150,7 +150,6 @@ static inline void _rcu_defer_queue(void (*fct)(void *p), void *p)
 		assert(head - LOAD_SHARED(defer_queue.tail) == 0);
 	}
 
-	smp_wmb();	/* Publish new pointer before write q[] */
 	if (unlikely(defer_queue.last_fct_in != fct)) {
 		defer_queue.last_fct_in = fct;
 		if (unlikely(DQ_IS_FCT_BIT(fct) || fct == DQ_FCT_MARK)) {
@@ -181,7 +180,8 @@ static inline void _rcu_defer_queue(void (*fct)(void *p), void *p)
 		}
 	}
 	_STORE_SHARED(defer_queue.q[head++ & DEFER_QUEUE_MASK], p);
-	smp_wmb();	/* Write q[] before head. */
+	smp_wmb();	/* Publish new pointer before head */
+			/* Write q[] before head. */
 	STORE_SHARED(defer_queue.head, head);
 }