Commit-ID: 39953dfd40077c7480b1d5deb4d617e086b1c865
Gitweb: http://git.kernel.org/tip/39953dfd40077c7480b1d5deb4d617e086b1c865
Author: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
AuthorDate: Tue, 12 Aug 2014 10:47:48 -0700
Committer: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
CommitDate: Tue, 16 Sep 2014 10:08:03 -0700
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 <amit.shah@xxxxxxxxxx>
Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
Tested-by: Paul Gortmaker <paul.gortmaker@xxxxxxxxxxxxx>
---
kernel/rcu/tree_plugin.h | 3 ++-
1 file changed, 2 insertions(+), 1 deletion(-)
diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
index d5aec54..4ad63d8 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);
--
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