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rcu/nocb: Recheck lazy callbacks under the ->nocb_lock from shrinker

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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: Frederic Weisbecker <frederic@kernel.org>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
This commit is contained in:
Frederic Weisbecker 2023-03-29 18:02:02 +02:00 committed by Paul E. McKenney
parent 7625926086
commit b96a8b0b5b
1 changed files with 11 additions and 3 deletions
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14
kernel/rcu/tree_nocb.h
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@ -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);