Commit b96a8b0b authored by Frederic Weisbecker's avatar Frederic Weisbecker Committed by Paul E. McKenney
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rcu/nocb: Recheck lazy callbacks under the ->nocb_lock from shrinker 



The ->lazy_len is only checked locklessly. Recheck again under the
->nocb_lock to avoid spending more time on flushing/waking if not
necessary. The ->lazy_len can still increment concurrently (from 1 to
infinity) but under the ->nocb_lock we at least know for sure if there
are lazy callbacks at all (->lazy_len > 0).

Signed-off-by: default avatarFrederic Weisbecker <frederic@kernel.org>
Signed-off-by: default avatarPaul E. McKenney <paulmck@kernel.org>
parent 76259260

kernel/rcu/tree_nocb.h

+11 −3
Original line number Diff line number Diff line
@@ -1358,12 +1358,20 @@ lazy_rcu_shrink_scan(struct shrinker *shrink, struct shrink_control *sc) 
		if (!rcu_rdp_is_offloaded(rdp))

			continue;



		_count = READ_ONCE(rdp->lazy_len);



		if (_count == 0)

		if (!READ_ONCE(rdp->lazy_len))

			continue;



		rcu_nocb_lock_irqsave(rdp, flags);

		/*

		 * Recheck under the nocb lock. Since we are not holding the bypass

		 * lock we may still race with increments from the enqueuer but still

		 * we know for sure if there is at least one lazy callback.

		 */

		_count = READ_ONCE(rdp->lazy_len);

		if (!_count) {

			rcu_nocb_unlock_irqrestore(rdp, flags);

			continue;

		}

		WARN_ON_ONCE(!rcu_nocb_flush_bypass(rdp, NULL, jiffies, false));

		rcu_nocb_unlock_irqrestore(rdp, flags);

		wake_nocb_gp(rdp, false);