netdev - [PATCH net v3] net: mctp: Fix tx queue stall

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Message-ID: <20251027065530.2045724-1-jinliangw@google.com>
Date: Sun, 26 Oct 2025 23:55:30 -0700
From: Jinliang Wang <jinliangw@...gle.com>
To: Jeremy Kerr <jk@...econstruct.com.au>, Matt Johnston <matt@...econstruct.com.au>, 
	netdev@...r.kernel.org
Cc: Andrew Lunn <andrew+netdev@...n.ch>, "David S. Miller" <davem@...emloft.net>, 
	Eric Dumazet <edumazet@...gle.com>, Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>, 
	linux-kernel@...r.kernel.org, Jinliang Wang <jinliangw@...gle.com>
Subject: [PATCH net v3] net: mctp: Fix tx queue stall

The tx queue can become permanently stuck in a stopped state due to a
race condition between the URB submission path and its completion
callback.

The URB completion callback can run immediately after usb_submit_urb()
returns, before the submitting function calls netif_stop_queue(). If
this occurs, the queue state management becomes desynchronized, leading
to a stall where the queue is never woken.

Fix this by moving the netif_stop_queue() call to before submitting the
URB. This closes the race window by ensuring the network stack is aware
the queue is stopped before the URB completion can possibly run.

Fixes: 0791c0327a6e ("net: mctp: Add MCTP USB transport driver")
Signed-off-by: Jinliang Wang <jinliangw@...gle.com>
---
v3:
 - target net tree, add fixes tag

v2:
 - remove duplicate comment in commit message

---
 drivers/net/mctp/mctp-usb.c | 8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)

diff --git a/drivers/net/mctp/mctp-usb.c b/drivers/net/mctp/mctp-usb.c
index 36ccc53b1797..ef860cfc629f 100644
--- a/drivers/net/mctp/mctp-usb.c
+++ b/drivers/net/mctp/mctp-usb.c
@@ -96,11 +96,13 @@ static netdev_tx_t mctp_usb_start_xmit(struct sk_buff *skb,
 			  skb->data, skb->len,
 			  mctp_usb_out_complete, skb);
 
+	/* Stops TX queue first to prevent race condition with URB complete */
+	netif_stop_queue(dev);
 	rc = usb_submit_urb(urb, GFP_ATOMIC);
-	if (rc)
+	if (rc) {
+		netif_wake_queue(dev);
 		goto err_drop;
-	else
-		netif_stop_queue(dev);
+	}
 
 	return NETDEV_TX_OK;
 
-- 
2.51.1.821.gb6fe4d2222-goog