mptcp: fix potential wake-up event loss

After the blamed commit below, the send buffer auto-tuning can
happen after that the mptcp_propagate_sndbuf() completes - via
the delegated action infrastructure.

We must check for write space even after such change or we risk
missing the wake-up event.

Fixes: 8005184f ("mptcp: refactor sndbuf auto-tuning")
Cc: stable@vger.kernel.org
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Reviewed-by: Mat Martineau <martineau@kernel.org>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Link: https://lore.kernel.org/r/20240223-upstream-net-20240223-misc-fixes-v1-6-162e87e48497@kernel.orgSigned-off-by: Jakub Kicinski <kuba@kernel.org>

net/mptcp/protocol.h

@@ -790,6 +790,16 @@ static inline bool mptcp_data_fin_enabled(const struct mptcp_sock *msk)
 	       READ_ONCE(msk->write_seq) == READ_ONCE(msk->snd_nxt);
 }
 
+static inline void mptcp_write_space(struct sock *sk)
+{
+	if (sk_stream_is_writeable(sk)) {
+		/* pairs with memory barrier in mptcp_poll */
+		smp_mb();
+		if (test_and_clear_bit(MPTCP_NOSPACE, &mptcp_sk(sk)->flags))
+			sk_stream_write_space(sk);
+	}
+}
+
 static inline void __mptcp_sync_sndbuf(struct sock *sk)
 {
 	struct mptcp_subflow_context *subflow;
@@ -808,6 +818,7 @@ static inline void __mptcp_sync_sndbuf(struct sock *sk)
 
 	/* the msk max wmem limit is <nr_subflows> * tcp wmem[2] */
 	WRITE_ONCE(sk->sk_sndbuf, new_sndbuf);
+	mptcp_write_space(sk);
 }
 
 /* The called held both the msk socket and the subflow socket locks,
@@ -838,16 +849,6 @@ static inline void mptcp_propagate_sndbuf(struct sock *sk, struct sock *ssk)
 	local_bh_enable();
 }
 
-static inline void mptcp_write_space(struct sock *sk)
-{
-	if (sk_stream_is_writeable(sk)) {
-		/* pairs with memory barrier in mptcp_poll */
-		smp_mb();
-		if (test_and_clear_bit(MPTCP_NOSPACE, &mptcp_sk(sk)->flags))
-			sk_stream_write_space(sk);
-	}
-}
-
 void mptcp_destroy_common(struct mptcp_sock *msk, unsigned int flags);
 
 #define MPTCP_TOKEN_MAX_RETRIES	4