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 <paulmck@linux.vnet.ibm.com>
Tested-by: Krister Johansen <kjlx@templeofstupid.com>
Cc: <stable@vger.kernel.org> # 4.6.x

diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
index 0b10425..573fbe9 100644 (file)
--- 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 @@ 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)