From: Paul E. McKenney Date: Tue, 12 Aug 2014 20:54:21 +0000 (-0700) Subject: rcu: Avoid misordering in nocb_leader_wait() X-Git-Tag: firefly_0821_release~176^2~3065^2~1^2~16^3 X-Git-Url: http://demsky.eecs.uci.edu/git/?a=commitdiff_plain;h=c847f14217d5aec5336272a54a32ffcf6e06ddcb;p=firefly-linux-kernel-4.4.55.git 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 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 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.