linux-kernel - [RFC PATCH-tip/locking/core v3 01/10] locking/osq: Make lock/unlock proper acquire/release barrier

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Date:	Fri, 17 Jun 2016 11:41:27 -0400
From:	Waiman Long <Waiman.Long@....com>
To:	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...hat.com>
Cc:	linux-kernel@...r.kernel.org, x86@...nel.org,
	linux-alpha@...r.kernel.org, linux-ia64@...r.kernel.org,
	linux-s390@...r.kernel.org, linux-arch@...r.kernel.org,
	linux-doc@...r.kernel.org, Davidlohr Bueso <dave@...olabs.net>,
	Jason Low <jason.low2@...com>,
	Dave Chinner <david@...morbit.com>,
	Jonathan Corbet <corbet@....net>,
	Scott J Norton <scott.norton@....com>,
	Douglas Hatch <doug.hatch@....com>,
	Waiman Long <Waiman.Long@....com>
Subject: [RFC PATCH-tip/locking/core v3 01/10] locking/osq: Make lock/unlock proper acquire/release barrier

The osq_lock() and osq_unlock() function may not provide the necessary
acquire and release barrier in some cases. This patch makes sure
that the proper barriers are provided when osq_lock() is successful
or when osq_unlock() is called.

The change on the unlock side is more for documentation purpose than
is actually needed.

Suggested-by: Peter Zijlstra (Intel) <peterz@...radead.org>
Signed-off-by: Waiman Long <Waiman.Long@....com>
---
 kernel/locking/osq_lock.c |    7 ++++++-
 1 files changed, 6 insertions(+), 1 deletions(-)

diff --git a/kernel/locking/osq_lock.c b/kernel/locking/osq_lock.c
index 05a3785..d957b90 100644
--- a/kernel/locking/osq_lock.c
+++ b/kernel/locking/osq_lock.c
@@ -124,6 +124,11 @@ bool osq_lock(struct optimistic_spin_queue *lock)
 
 		cpu_relax_lowlatency();
 	}
+	/*
+	 * Add an acquire memory barrier for pairing with the release barrier
+	 * in unlock.
+	 */
+	smp_acquire__after_ctrl_dep();
 	return true;
 
 unqueue:
@@ -198,7 +203,7 @@ void osq_unlock(struct optimistic_spin_queue *lock)
 	 * Second most likely case.
 	 */
 	node = this_cpu_ptr(&osq_node);
-	next = xchg(&node->next, NULL);
+	next = xchg_release(&node->next, NULL);
 	if (next) {
 		WRITE_ONCE(next->locked, 1);
 		return;
-- 
1.7.1