rcu: Add memory barriers for NOCB leader wakeup

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 <[email protected]> Tested-by: Krister Johansen <[email protected]> Cc: <[email protected]> # 4.6.x
olifre · Jun 9, 2017 · 6b5fc3a · 6b5fc3a
1 parent 511324e
commit 6b5fc3a
Showing 1 changed file with 2 additions and 0 deletions.
diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
@@ -1810,6 +1810,7 @@ static void wake_nocb_leader(struct rcu_data *rdp, bool force)
 	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);
 	}
 }
@@ -2064,6 +2065,7 @@ static void nocb_leader_wait(struct rcu_data *my_rdp)
 	 * 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)