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@google.com>
Reviewed-by: David Ahern <dsahern@kernel.org>
Acked-by: Neal Cardwell <ncardwell@google.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>

diff --git a/net/ipv4/tcp_metrics.c b/net/ipv4/tcp_metrics.c
index 7aca12c59c18483f42276d01252ed0fac326e5d8..c2a925538542b5d787596b7d76705dda86cf48d8 100644 (file)
--- 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();
        }
 }