mptcp: defer work schedule until mptcp lock is released

Don't schedule the work queue right away, instead defer this
to the lock release callback.

This has the advantage that it will give recv path a chance to
complete -- this might have moved all pending packets from the
subflow to the mptcp receive queue, which allows to avoid the
schedule_work().
Co-developed-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Reviewed-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/mptcp/protocol.c

@@ -238,9 +238,16 @@ void mptcp_data_ready(struct sock *sk, struct sock *ssk)
 	if (atomic_read(&sk->sk_rmem_alloc) > READ_ONCE(sk->sk_rcvbuf))
 		goto wake;
 
-	if (schedule_work(&msk->work))
-		sock_hold((struct sock *)msk);
+	/* mptcp socket is owned, release_cb should retry */
+	if (!test_and_set_bit(TCP_DELACK_TIMER_DEFERRED,
+			      &sk->sk_tsq_flags)) {
+		sock_hold(sk);
 
+		/* need to try again, its possible release_cb() has already
+		 * been called after the test_and_set_bit() above.
+		 */
+		move_skbs_to_msk(msk, ssk);
+	}
 wake:
 	sk->sk_data_ready(sk);
 }
@@ -941,6 +948,32 @@ static int mptcp_getsockopt(struct sock *sk, int level, int optname,
 	return -EOPNOTSUPP;
 }
 
+#define MPTCP_DEFERRED_ALL TCPF_DELACK_TIMER_DEFERRED
+
+/* this is very alike tcp_release_cb() but we must handle differently a
+ * different set of events
+ */
+static void mptcp_release_cb(struct sock *sk)
+{
+	unsigned long flags, nflags;
+
+	do {
+		flags = sk->sk_tsq_flags;
+		if (!(flags & MPTCP_DEFERRED_ALL))
+			return;
+		nflags = flags & ~MPTCP_DEFERRED_ALL;
+	} while (cmpxchg(&sk->sk_tsq_flags, flags, nflags) != flags);
+
+	if (flags & TCPF_DELACK_TIMER_DEFERRED) {
+		struct mptcp_sock *msk = mptcp_sk(sk);
+		struct sock *ssk;
+
+		ssk = mptcp_subflow_recv_lookup(msk);
+		if (!ssk || !schedule_work(&msk->work))
+			__sock_put(sk);
+	}
+}
+
 static int mptcp_get_port(struct sock *sk, unsigned short snum)
 {
 	struct mptcp_sock *msk = mptcp_sk(sk);
@@ -1016,6 +1049,7 @@ static struct proto mptcp_prot = {
 	.destroy	= mptcp_destroy,
 	.sendmsg	= mptcp_sendmsg,
 	.recvmsg	= mptcp_recvmsg,
+	.release_cb	= mptcp_release_cb,
 	.hash		= inet_hash,
 	.unhash		= inet_unhash,
 	.get_port	= mptcp_get_port,