Commit-ID: c847f14217d5aec5336272a54a32ffcf6e06ddcb
Gitweb: http://git.kernel.org/tip/c847f14217d5aec5336272a54a32ffcf6e06ddcb
Author: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
AuthorDate: Tue, 12 Aug 2014 13:54:21 -0700
Committer: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
CommitDate: Tue, 16 Sep 2014 10:08:03 -0700
rcu: Avoid misordering in nocb_leader_wait()
The NOCB follower wakeup ordering depends on the store to the tail
pointer happening before the wakeup. However, because atomic_long_add()
does not return a value, it does not provide ordering guarantees, and
the locking in wake_up() only guarantees that the store will happen
before the unlock, which might be too late. Even though this is only a
theoretical issue, this commit 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 | 1 +
1 file changed, 1 insertion(+)
diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
index 8b73518..c554acc 100644
--- a/kernel/rcu/tree_plugin.h
+++ b/kernel/rcu/tree_plugin.h
@@ -2306,6 +2306,7 @@ wait_again:
atomic_long_add(rdp->nocb_gp_count, &rdp->nocb_follower_count);
atomic_long_add(rdp->nocb_gp_count_lazy,
&rdp->nocb_follower_count_lazy);
+ smp_mb__after_atomic(); /* Store *tail before wakeup. */
if (rdp != my_rdp && tail == &rdp->nocb_follower_head) {
/*
* List was empty, wake up the follower.
--
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