linux-kernel - [PATCH 4.12 58/84] rcu: Add memory barriers for NOCB leader wakeup

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Date:   Wed, 19 Jul 2017 11:44:04 +0200
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     linux-kernel@...r.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        stable@...r.kernel.org,
        "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
        Krister Johansen <kjlx@...pleofstupid.com>
Subject: [PATCH 4.12 58/84] rcu: Add memory barriers for NOCB leader wakeup

4.12-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>

commit 6b5fc3a1331810db407c9e0e673dc1837afdc9d0 upstream.

Wait/wakeup operations do not guarantee ordering on their own.  Instead,
either locking or memory barriers are required.  This commit therefore
adds memory barriers to wake_nocb_leader() and nocb_leader_wait().

Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
Tested-by: Krister Johansen <kjlx@...pleofstupid.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>

---
 kernel/rcu/tree_plugin.h |    2 ++
 1 file changed, 2 insertions(+)

--- a/kernel/rcu/tree_plugin.h
+++ b/kernel/rcu/tree_plugin.h
@@ -1769,6 +1769,7 @@ static void wake_nocb_leader(struct rcu_
 	if (READ_ONCE(rdp_leader->nocb_leader_sleep) || force) {
 		/* Prior smp_mb__after_atomic() orders against prior enqueue. */
 		WRITE_ONCE(rdp_leader->nocb_leader_sleep, false);
+		smp_mb(); /* ->nocb_leader_sleep before swake_up(). */
 		swake_up(&rdp_leader->nocb_wq);
 	}
 }
@@ -2023,6 +2024,7 @@ wait_again:
 	 * nocb_gp_head, where they await a grace period.
 	 */
 	gotcbs = false;
+	smp_mb(); /* wakeup before ->nocb_head reads. */
 	for (rdp = my_rdp; rdp; rdp = rdp->nocb_next_follower) {
 		rdp->nocb_gp_head = READ_ONCE(rdp->nocb_head);
 		if (!rdp->nocb_gp_head)