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From:   Frederic Weisbecker <frederic@kernel.org>
To:     "Paul E . McKenney" <paulmck@kernel.org>
Cc:     LKML <linux-kernel@vger.kernel.org>,
        Frederic Weisbecker <frederic@kernel.org>,
        rcu <rcu@vger.kernel.org>, Uladzislau Rezki <urezki@gmail.com>,
        Neeraj Upadhyay <quic_neeraju@quicinc.com>,
        Boqun Feng <boqun.feng@gmail.com>,
        Joel Fernandes <joel@joelfernandes.org>
Subject: [PATCH 3/4] rcu/nocb: Recheck lazy callbacks under the ->nocb_lock from shrinker
Date:   Wed, 22 Mar 2023 20:44:55 +0100
Message-Id: <20230322194456.2331527-4-frederic@kernel.org>
In-Reply-To: <20230322194456.2331527-1-frederic@kernel.org>
References: <20230322194456.2331527-1-frederic@kernel.org>
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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>
---
 kernel/rcu/tree_nocb.h | 16 ++++++++++++----
 1 file changed, 12 insertions(+), 4 deletions(-)

diff --git a/kernel/rcu/tree_nocb.h b/kernel/rcu/tree_nocb.h
index cb57e8312231..a3dc7465b0b2 100644
--- a/kernel/rcu/tree_nocb.h
+++ b/kernel/rcu/tree_nocb.h
@@ -1350,12 +1350,20 @@ lazy_rcu_shrink_scan(struct shrinker *shrink, struct shrink_control *sc)
 		if (!rcu_rdp_is_offloaded(rdp))
 			continue;
 
+		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 == 0)
+		if (!_count) {
+			rcu_nocb_unlock_irqrestore(rdp, flags);
 			continue;
-
-		rcu_nocb_lock_irqsave(rdp, flags);
+		}
 		WARN_ON_ONCE(!rcu_nocb_flush_bypass(rdp, NULL, jiffies, false));
 		rcu_nocb_unlock_irqrestore(rdp, flags);
 		wake_nocb_gp(rdp, false);
-- 
2.34.1