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authorPaul E. McKenney <paulmck@linux.vnet.ibm.com>2014-08-12 22:54:21 +0200
committerPaul E. McKenney <paulmck@linux.vnet.ibm.com>2014-09-16 19:08:03 +0200
commitc847f14217d5aec5336272a54a32ffcf6e06ddcb (patch)
tree865d138fe6a772c37c4928518b257bb14834c793 /kernel/rcu
parentrcu: Handle NOCB callbacks from irq-disabled idle code (diff)
downloadlinux-c847f14217d5aec5336272a54a32ffcf6e06ddcb.tar.xz
linux-c847f14217d5aec5336272a54a32ffcf6e06ddcb.zip
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@redhat.com> Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Tested-by: Paul Gortmaker <paul.gortmaker@windriver.com>
Diffstat (limited to 'kernel/rcu')
-rw-r--r--kernel/rcu/tree_plugin.h1
1 files changed, 1 insertions, 0 deletions
diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
index 8b7351836228..c554accfc5f5 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.