rcu/nocb: Fix shrinker race against callback enqueuer

The shrinker resets the lazy callbacks counter in order to trigger the
pending lazy queue flush though the rcuog kthread. The counter reset is
protected by the ->nocb_lock against concurrent accesses...except
for one of them. Here is a list of existing synchronized readers/writer:

1) The first lazy enqueuer (incrementing ->lazy_len to 1) does so under
   ->nocb_lock and ->nocb_bypass_lock.

2) The further lazy enqueuers (incrementing ->lazy_len above 1) do so
   under ->nocb_bypass_lock _only_.

3) The lazy flush checks and resets to 0 under ->nocb_lock and
	->nocb_bypass_lock.

The shrinker protects its ->lazy_len reset against cases 1) and 3) but
not against 2). As such, setting ->lazy_len to 0 under the ->nocb_lock
may be cancelled right away by an overwrite from an enqueuer, leading
rcuog to ignore the flush.

To avoid that, use the proper bypass flush API which takes care of all
those details.

Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

diff --git a/kernel/rcu/tree_nocb.h b/kernel/rcu/tree_nocb.h
index 1a86883902ce549cfdd7a7c0a42a364236889da9..c321fce2af8e31b493c38826e5dd4a9eb121f4be 100644 (file)
--- a/kernel/rcu/tree_nocb.h
+++ b/kernel/rcu/tree_nocb.h
@@ -1364,7 +1364,7 @@ lazy_rcu_shrink_scan(struct shrinker *shrink, struct shrink_control *sc)
                        continue;
 
                rcu_nocb_lock_irqsave(rdp, flags);
-               WRITE_ONCE(rdp->lazy_len, 0);
+               WARN_ON_ONCE(!rcu_nocb_flush_bypass(rdp, NULL, jiffies, false));
                rcu_nocb_unlock_irqrestore(rdp, flags);
                wake_nocb_gp(rdp, false);
                sc->nr_to_scan -= _count;