tcp: add a fast path in tcp_delack_timer()

delack timer is not stopped from inet_csk_clear_xmit_timer()
because we do not define INET_CSK_CLEAR_TIMERS.

This is a conscious choice : inet_csk_clear_xmit_timer()
is often called from another cpu. Calling del_timer()
would cause false sharing and lock contention.

This means that very often, tcp_delack_timer() is called
at the timer expiration, while there is no ACK to transmit.

This can be detected very early, avoiding the socket spinlock.

Notes:
- test about tp->compressed_ack is racy,
  but in the unlikely case there is a race, the dedicated
  compressed_ack_timer hrtimer would close it.

- Even if the fast path is not taken, reading
  icsk->icsk_ack.pending and tp->compressed_ack
  before acquiring the socket spinlock reduces
  acquisition time and chances of contention.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Link: https://patch.msgid.link/20241002173042.917928-4-edumazet@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

1 files changed, 2 insertions, 2 deletions

diff --git a/net/ipv4/inet_connection_sock.c b/net/ipv4/inet_connection_sock.c
index 8c53385cc808..12e975ed4910 100644
--- a/net/ipv4/inet_connection_sock.c
+++ b/net/ipv4/inet_connection_sock.c
@@ -776,7 +776,7 @@ void inet_csk_clear_xmit_timers(struct sock *sk)
 	struct inet_connection_sock *icsk = inet_csk(sk);
 
 	smp_store_release(&icsk->icsk_pending, 0);
-	icsk->icsk_ack.pending = 0;
+	smp_store_release(&icsk->icsk_ack.pending, 0);
 
 	sk_stop_timer(sk, &icsk->icsk_retransmit_timer);
 	sk_stop_timer(sk, &icsk->icsk_delack_timer);
@@ -792,7 +792,7 @@ void inet_csk_clear_xmit_timers_sync(struct sock *sk)
 	sock_not_owned_by_me(sk);
 
 	smp_store_release(&icsk->icsk_pending, 0);
-	icsk->icsk_ack.pending = 0;
+	smp_store_release(&icsk->icsk_ack.pending, 0);
 
 	sk_stop_timer_sync(sk, &icsk->icsk_retransmit_timer);
 	sk_stop_timer_sync(sk, &icsk->icsk_delack_timer);