Later patches need to ensure that all MPJ subflows are grafted to the
msk socket before accept() completion.

Currently the grafting happens under the msk socket lock: potentially
at msk release_cb time which make satisfying the above condition a bit
tricky.

Move the MPJ subflow grafting earlier, under the msk data lock, so that
we can use such lock as a synchronization point.

Signed-off-by: Paolo Abeni <pabeni@redhat.com>
---
v3 -> v4:
  - clarified it's not a fix
  - move the graft under the msk socket lock
  - no need to graft for active subflows
---
 net/mptcp/protocol.c | 30 +++++++++++++++++++++++-------
 1 file changed, 23 insertions(+), 7 deletions(-)

diff --git a/net/mptcp/protocol.c b/net/mptcp/protocol.c
index 78ac8ba80e59..4a4cb9952596 100644
--- a/net/mptcp/protocol.c
+++ b/net/mptcp/protocol.c
@@ -933,12 +933,6 @@ static bool __mptcp_finish_join(struct mptcp_sock *msk, struct sock *ssk)
 	mptcp_subflow_joined(msk, ssk);
 	spin_unlock_bh(&msk->fallback_lock);
 
-	/* attach to msk socket only after we are sure we will deal with it
-	 * at close time
-	 */
-	if (sk->sk_socket && !ssk->sk_socket)
-		mptcp_sock_graft(ssk, sk->sk_socket);
-
 	mptcp_subflow_ctx(ssk)->subflow_id = msk->subflow_id++;
 	mptcp_sockopt_sync_locked(msk, ssk);
 	mptcp_stop_tout_timer(sk);
@@ -3760,6 +3754,20 @@ void mptcp_sock_graft(struct sock *sk, struct socket *parent)
 	write_unlock_bh(&sk->sk_callback_lock);
 }
 
+/* Can be called without holding the msk socket lock; use the callback lock
+ * to avoid {READ_,WRITE_}ONCE annotations on sk_socket.
+ */
+static void mptcp_sock_check_graft(struct sock *sk, struct sock *ssk)
+{
+	struct socket *sock;
+
+	write_lock_bh(&sk->sk_callback_lock);
+	sock = sk->sk_socket;
+	write_unlock_bh(&sk->sk_callback_lock);
+	if (sock)
+		mptcp_sock_graft(ssk, sock);
+}
+
 bool mptcp_finish_join(struct sock *ssk)
 {
 	struct mptcp_subflow_context *subflow = mptcp_subflow_ctx(ssk);
@@ -3775,7 +3783,9 @@ bool mptcp_finish_join(struct sock *ssk)
 		return false;
 	}
 
-	/* active subflow, already present inside the conn_list */
+	/* Active subflow, already present inside the conn_list; is grafted
+	 * either by __mptcp_subflow_connect() or accept.
+	 */
 	if (!list_empty(&subflow->node)) {
 		spin_lock_bh(&msk->fallback_lock);
 		if (!msk->allow_subflows) {
@@ -3802,11 +3812,17 @@ bool mptcp_finish_join(struct sock *ssk)
 		if (ret) {
 			sock_hold(ssk);
 			list_add_tail(&subflow->node, &msk->conn_list);
+			mptcp_sock_check_graft(parent, ssk);
 		}
 	} else {
 		sock_hold(ssk);
 		list_add_tail(&subflow->node, &msk->join_list);
 		__set_bit(MPTCP_FLUSH_JOIN_LIST, &msk->cb_flags);
+
+		/* In case of later failures, __mptcp_flush_join_list() will
+		 * properly orphan the ssk via mptcp_close_ssk().
+		 */
+		mptcp_sock_check_graft(parent, ssk);
 	}
 	mptcp_data_unlock(parent);
 
-- 
2.51.1