From: Paul E. McKenney Date: Tue, 12 Aug 2014 17:47:48 +0000 (-0700) Subject: rcu: Avoid misordering in __call_rcu_nocb_enqueue() X-Git-Tag: firefly_0821_release~176^2~3065^2~1^2~16^3~2 X-Git-Url: http://demsky.eecs.uci.edu/git/?a=commitdiff_plain;h=39953dfd40077c7480b1d5deb4d617e086b1c865;p=firefly-linux-kernel-4.4.55.git rcu: Avoid misordering in __call_rcu_nocb_enqueue() The NOCB leader wakeup ordering depends on the store to the header happening before the check for the leader already being awake. However, because atomic_long_add() does not return a value, it does not provide ordering guarantees, the incorrect comment in wake_nocb_leader() notwithstanding. This commit therefore adds a smp_mb__after_atomic() after the final atomic_long_add() to provide the needed ordering guarantee. Reported-by: Amit Shah Signed-off-by: Paul E. McKenney Tested-by: Paul Gortmaker --- diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h index d5aec549558d..4ad63d861599 100644 --- a/kernel/rcu/tree_plugin.h +++ b/kernel/rcu/tree_plugin.h @@ -2042,7 +2042,7 @@ static void wake_nocb_leader(struct rcu_data *rdp, bool force) if (!ACCESS_ONCE(rdp_leader->nocb_kthread)) return; if (ACCESS_ONCE(rdp_leader->nocb_leader_sleep) || force) { - /* Prior xchg orders against prior callback enqueue. */ + /* Prior smp_mb__after_atomic() orders against prior enqueue. */ ACCESS_ONCE(rdp_leader->nocb_leader_sleep) = false; wake_up(&rdp_leader->nocb_wq); } @@ -2071,6 +2071,7 @@ static void __call_rcu_nocb_enqueue(struct rcu_data *rdp, ACCESS_ONCE(*old_rhpp) = rhp; atomic_long_add(rhcount, &rdp->nocb_q_count); atomic_long_add(rhcount_lazy, &rdp->nocb_q_count_lazy); + smp_mb__after_atomic(); /* Store *old_rhpp before _wake test. */ /* If we are not being polled and there is a kthread, awaken it ... */ t = ACCESS_ONCE(rdp->nocb_kthread);