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@google.com>
Acked-by: Jeremy Kerr <jk@codeconstruct.com.au>
Link: https://patch.msgid.link/20251027065530.2045724-1-jinliangw@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

diff --git a/drivers/net/mctp/mctp-usb.c b/drivers/net/mctp/mctp-usb.c
index 36ccc53b17975912c7d310fe8d58eb91fe482732..ef860cfc629f65dd3c08d0b817ad4ab11e18c42a 100644 (file)
--- 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;