mptcp: Do not traverse the subflow connection list without lock

The MPTCP socket's conn_list (list of subflows) requires the socket lock
to access. The MP_FAIL timeout code added such an access, where it would
check the list of subflows both in timer context and (later) in workqueue
context where the socket lock is held.

Rather than check the list twice, remove the check in the timeout
handler and only depend on the check in the workqueue. Also remove the
MPTCP_FAIL_NO_RESPONSE flag, since mptcp_mp_fail_no_response() has
insignificant overhead and can be checked on each worker run.

Fixes: 49fa1919d6 ("mptcp: reset subflow when MP_FAIL doesn't respond")
Reported-by: Paolo Abeni <pabeni@redhat.com>
Reviewed-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

net/mptcp/protocol.c

@@ -2190,23 +2190,10 @@ mp_fail_response_expect_subflow(struct mptcp_sock *msk)
 	return ret;
 }
 
-static void mptcp_check_mp_fail_response(struct mptcp_sock *msk)
-{
-	struct mptcp_subflow_context *subflow;
-	struct sock *sk = (struct sock *)msk;
-
-	bh_lock_sock(sk);
-	subflow = mp_fail_response_expect_subflow(msk);
-	if (subflow)
-		__set_bit(MPTCP_FAIL_NO_RESPONSE, &msk->flags);
-	bh_unlock_sock(sk);
-}
-
 static void mptcp_timeout_timer(struct timer_list *t)
 {
 	struct sock *sk = from_timer(sk, t, sk_timer);
 
-	mptcp_check_mp_fail_response(mptcp_sk(sk));
 	mptcp_schedule_work(sk);
 	sock_put(sk);
 }
@@ -2588,8 +2575,7 @@ static void mptcp_worker(struct work_struct *work)
 	if (test_and_clear_bit(MPTCP_WORK_RTX, &msk->flags))
 		__mptcp_retrans(sk);
 
-	if (test_and_clear_bit(MPTCP_FAIL_NO_RESPONSE, &msk->flags))
-		mptcp_mp_fail_no_response(msk);
+	mptcp_mp_fail_no_response(msk);
 
 unlock:
 	release_sock(sk);

net/mptcp/protocol.h

@@ -117,7 +117,6 @@
 #define MPTCP_WORK_EOF		3
 #define MPTCP_FALLBACK_DONE	4
 #define MPTCP_WORK_CLOSE_SUBFLOW 5
-#define MPTCP_FAIL_NO_RESPONSE	6
 
 /* MPTCP socket release cb flags */
 #define MPTCP_PUSH_PENDING	1