[PATCH 6.12 176/567] mptcp: schedule rtx timer only after pushing data

Greg Kroah-Hartman
2026-01-06T17:25:48Z

From: Paolo Abeni <pabeni@redhat.com>

commit 2ea6190f42d0416a4310e60a7fcb0b49fcbbd4fb upstream.

The MPTCP protocol usually schedule the retransmission timer only
when there is some chances for such retransmissions to happen.

With a notable exception: __mptcp_push_pending() currently schedule
such timer unconditionally, potentially leading to unnecessary rtx
timer expiration.

The issue is present since the blamed commit below but become easily
reproducible after commit 27b0e701d387 ("mptcp: drop bogus optimization
in __mptcp_check_push()")

Fixes: 33d41c9cd74c ("mptcp: more accurate timeout")
Cc: stable@vger.kernel.org
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Reviewed-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Link: https://patch.msgid.link/20251205-net-mptcp-misc-fixes-6-19-rc1-v1-3-9e4781a6c1b8@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
net/mptcp/protocol.c | 15 +++++++++------
1 file changed, 9 insertions(+), 6 deletions(-)

--- a/net/mptcp/protocol.c
+++ b/net/mptcp/protocol.c
@@ -1637,7 +1637,7 @@ void __mptcp_push_pending(struct sock *s
struct mptcp_sendmsg_info info = {
.flags = flags,
};
- bool do_check_data_fin = false;
+ bool copied = false;
int push_count = 1;

while (mptcp_send_head(sk) && (push_count > 0)) {
@@ -1679,7 +1679,7 @@ void __mptcp_push_pending(struct sock *s
push_count--;
continue;
}
- do_check_data_fin = true;
+ copied = true;
}
}
}
@@ -1688,11 +1688,14 @@ void __mptcp_push_pending(struct sock *s
if (ssk)
mptcp_push_release(ssk, &info);

- /* ensure the rtx timer is running */
- if (!mptcp_rtx_timer_pending(sk))
- mptcp_reset_rtx_timer(sk);
- if (do_check_data_fin)
+ /* Avoid scheduling the rtx timer if no data has been pushed; the timer
+ * will be updated on positive acks by __mptcp_cleanup_una().
+ */
+ if (copied) {
+ if (!mptcp_rtx_timer_pending(sk))
+ mptcp_reset_rtx_timer(sk);
mptcp_check_send_data_fin(sk);
+ }
}

static void __mptcp_subflow_push_pending(struct sock *sk, struct sock *ssk, bool first)