[PATCH mptcp-net v4 2/5] mptcp: fix the stall problems using backlog_queue

[Gang Yan <gang.yan@linux.dev>]

From: Gang Yan <yangang@kylinos.cn>

The original condition would stop moving skbs or spooling backlog even
when the receive queue is empty, leading to receive stall.

Modify the condition in __mptcp_move_skbs() and mptcp_can_spool_backlog()
to only treat rcvbuf as full when:
  sk_rmem_alloc_get(sk) > sk->sk_rcvbuf && !skb_queue_empty(&sk->sk_receive_queue)

This ensures the backlog can still be moved to the receive queue when
the queue is empty, avoiding stall problems.

Fixes: 6228efe0cc01 ("mptcp: leverage the backlog for RX packet processing")
Co-developed-by: Geliang Tang <geliang@kernel.org>
Signed-off-by: Geliang Tang <geliang@kernel.org>
Signed-off-by: Gang Yan <yangang@kylinos.cn>
---
 net/mptcp/protocol.c | 10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

diff --git a/net/mptcp/protocol.c b/net/mptcp/protocol.c
index 03eaa92badd5..01d04cbea53d 100644
--- a/net/mptcp/protocol.c
+++ b/net/mptcp/protocol.c
@@ -2197,6 +2197,12 @@ static void mptcp_rcv_space_adjust(struct mptcp_sock *msk, int copied)
 	msk->rcvq_space.time = mstamp;
 }
 
+static bool mptcp_can_dequeue_from_backlog(struct sock *sk)
+{
+	return sk_rmem_alloc_get(sk) <= sk->sk_rcvbuf ||
+	       skb_queue_empty(&sk->sk_receive_queue);
+}
+
 static bool __mptcp_move_skbs(struct sock *sk, struct list_head *skbs, u32 *delta)
 {
 	struct sk_buff *skb = list_first_entry(skbs, struct sk_buff, list);
@@ -2206,7 +2212,7 @@ static bool __mptcp_move_skbs(struct sock *sk, struct list_head *skbs, u32 *delt
 	*delta = 0;
 	while (1) {
 		/* If the msk recvbuf is full stop, don't drop */
-		if (sk_rmem_alloc_get(sk) > sk->sk_rcvbuf)
+		if (!mptcp_can_dequeue_from_backlog(sk))
 			break;
 
 		prefetch(skb->next);
@@ -2261,7 +2267,7 @@ static bool mptcp_can_spool_backlog(struct sock *sk, struct list_head *skbs)
 
 	/* Don't spool the backlog if the rcvbuf is full. */
 	if (RB_EMPTY_ROOT(&msk->backlog_queue) ||
-	    sk_rmem_alloc_get(sk) > sk->sk_rcvbuf)
+	    !mptcp_can_dequeue_from_backlog(sk))
 		return false;
 
 	INIT_LIST_HEAD(skbs);
-- 
2.43.0