netdev - [PATCH net-next 4/4] tcp_metrics: optimize tcp_metrics_flush

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Date: Fri, 22 Sep 2023 22:03:56 +0000
From: Eric Dumazet <edumazet@...gle.com>
To: "David S . Miller" <davem@...emloft.net>, Jakub Kicinski <kuba@...nel.org>, 
	Paolo Abeni <pabeni@...hat.com>
Cc: Neal Cardwell <ncardwell@...gle.com>, Yuchung Cheng <ycheng@...gle.com>, netdev@...r.kernel.org, 
	eric.dumazet@...il.com, Eric Dumazet <edumazet@...gle.com>
Subject: [PATCH net-next 4/4] tcp_metrics: optimize tcp_metrics_flush_all()

This is inspired by several syzbot reports where
tcp_metrics_flush_all() was seen in the traces.

We can avoid acquiring tcp_metrics_lock for empty buckets,
and we should add one cond_resched() to break potential long loops.

Signed-off-by: Eric Dumazet <edumazet@...gle.com>
---
 net/ipv4/tcp_metrics.c | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/net/ipv4/tcp_metrics.c b/net/ipv4/tcp_metrics.c
index 7aca12c59c18483f42276d01252ed0fac326e5d8..c2a925538542b5d787596b7d76705dda86cf48d8 100644
--- a/net/ipv4/tcp_metrics.c
+++ b/net/ipv4/tcp_metrics.c
@@ -898,11 +898,13 @@ static void tcp_metrics_flush_all(struct net *net)
 	unsigned int row;
 
 	for (row = 0; row < max_rows; row++, hb++) {
-		struct tcp_metrics_block __rcu **pp;
+		struct tcp_metrics_block __rcu **pp = &hb->chain;
 		bool match;
 
+		if (!rcu_access_pointer(*pp))
+			continue;
+
 		spin_lock_bh(&tcp_metrics_lock);
-		pp = &hb->chain;
 		for (tm = deref_locked(*pp); tm; tm = deref_locked(*pp)) {
 			match = net ? net_eq(tm_net(tm), net) :
 				!refcount_read(&tm_net(tm)->ns.count);
@@ -914,6 +916,7 @@ static void tcp_metrics_flush_all(struct net *net)
 			}
 		}
 		spin_unlock_bh(&tcp_metrics_lock);
+		cond_resched();
 	}
 }
 
-- 
2.42.0.515.g380fc7ccd1-goog