[PATCH 6.1.y 2/6] mptcp: schedule rtx timer only after pushing data

["Matthieu Baerts (NGI0)" <matttbe@kernel.org>]

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>
[ Conflicts in protocol.c, because commit 0fa1b3783a17 ("mptcp: use
  get_send wrapper") is not in this version, and is changing the
  context. The same modification can still be applied. ]
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
---
 net/mptcp/protocol.c | 15 +++++++++------
 1 file changed, 9 insertions(+), 6 deletions(-)

diff --git a/net/mptcp/protocol.c b/net/mptcp/protocol.c
index 2d107a1f2ef9..ad0bfdd308be 100644
--- a/net/mptcp/protocol.c
+++ b/net/mptcp/protocol.c
@@ -1666,7 +1666,7 @@ void __mptcp_push_pending(struct sock *sk, unsigned int flags)
 	struct mptcp_sendmsg_info info = {
 				.flags = flags,
 	};
-	bool do_check_data_fin = false;
+	bool copied = false;
 	struct mptcp_data_frag *dfrag;
 	int len;
 
@@ -1703,7 +1703,7 @@ void __mptcp_push_pending(struct sock *sk, unsigned int flags)
 				goto out;
 			}
 
-			do_check_data_fin = true;
+			copied = true;
 			info.sent += ret;
 			len -= ret;
 
@@ -1717,11 +1717,14 @@ void __mptcp_push_pending(struct sock *sk, unsigned int flags)
 		mptcp_push_release(ssk, &info);
 
 out:
-	/* 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)
-- 
2.51.0