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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>