mptcp: stop using the mptcp_has_another_subflow() helper

The mentioned helper requires the msk socket lock, and the
current callers don't own it nor can't acquire it, so the
access is racy.

All the current callers are really checking for infinite mapping
fallback, and the latter condition is explicitly tracked by
the relevant msk variable: we can safely remove the caller usage
- and the caller itself.

The issue is present since MP_FAIL implementation, but the
fix only applies since the infinite fallback support, ence the
somewhat unexpected fixes tag.

Fixes: 0530020a ("mptcp: track and update contiguous data status")
Acked-and-tested-by: Geliang Tang <geliang.tang@suse.com>
Signed-off-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/pm.c

@@ -303,7 +303,7 @@ void mptcp_pm_mp_fail_received(struct sock *sk, u64 fail_seq)
 
 	pr_debug("fail_seq=%llu", fail_seq);
 
-	if (mptcp_has_another_subflow(sk) || !READ_ONCE(msk->allow_infinite_fallback))
+	if (!READ_ONCE(msk->allow_infinite_fallback))
 		return;
 
 	if (!READ_ONCE(subflow->mp_fail_response_expect)) {

net/mptcp/protocol.h

@@ -650,19 +650,6 @@ static inline void mptcp_subflow_tcp_fallback(struct sock *sk,
 	inet_csk(sk)->icsk_af_ops = ctx->icsk_af_ops;
 }
 
-static inline bool mptcp_has_another_subflow(struct sock *ssk)
-{
-	struct mptcp_subflow_context *subflow = mptcp_subflow_ctx(ssk), *tmp;
-	struct mptcp_sock *msk = mptcp_sk(subflow->conn);
-
-	mptcp_for_each_subflow(msk, tmp) {
-		if (tmp != subflow)
-			return true;
-	}
-
-	return false;
-}
-
 void __init mptcp_proto_init(void);
 #if IS_ENABLED(CONFIG_MPTCP_IPV6)
 int __init mptcp_proto_v6_init(void);

net/mptcp/subflow.c

@@ -1233,8 +1233,7 @@ static bool subflow_check_data_avail(struct sock *ssk)
 	if (!__mptcp_check_fallback(msk)) {
 		/* RFC 8684 section 3.7. */
 		if (subflow->send_mp_fail) {
-			if (mptcp_has_another_subflow(ssk) ||
-			    !READ_ONCE(msk->allow_infinite_fallback)) {
+			if (!READ_ONCE(msk->allow_infinite_fallback)) {
 				ssk->sk_err = EBADMSG;
 				tcp_set_state(ssk, TCP_CLOSE);
 				subflow->reset_transient = 0;