Re: [PATCH] usb-gadget-ether: Prevent oops caused by error interrupt race -V2 (comments update)

Thomas Gleixner
2007-09-22T20:23:16Z

On Sat, 2007-09-22 at 13:14 -0700, David Brownell wrote:
> How's this? Note that the queue should already have been stopped,
> so I removed what should be an extra call (as well as fixing the
> comments).

Yeah, stop queue should be not necessary.

> - Dave
>
> ========
> From: Thomas Gleixner <tglx@linutronix.de>

Please change to:

From: Benedikt Spranger <bene@linutronix.de>

He did all the grump work of figuring out what's going wrong. I was just
the messenger.

> This patch fixes a longstanding race in the Ethernet gadget driver,
> which can cause an oops on device disconnect. The fix is just to
> make the TX path check whether its freelist is empty. That check
> is otherwise not necessary, since the queue is always stopped when
> that list empties (and restarted when request completion puts an
> entry back on that freelist).

Sigh. I need a real deep look inside that code to understand, why
tx_reqs is not a requestlist but a freelist. Very intuitive naming :)

> The race window starts when the network code decides to transmit a
> packet, and ends when hard_start_xmit() grabs the freelist lock.
> If disconnect() is called inside that window, it shuts down the
> TX queue and breaks the otherwise-solid assumption that packets are
> never sent when the TX queue is stopped.

Please add our signed offs as well

Signed-off-by: Benedikt Spranger <bene@linutronix.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

> Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>

Thanks,
tglx

> --- a/drivers/usb/gadget/ether.c
> +++ b/drivers/usb/gadget/ether.c
> @@ -1989,8 +1989,20 @@ static int eth_start_xmit (struct sk_buff *skb, struct net_device *net)
> }
>
> spin_lock_irqsave(&dev->req_lock, flags);
> + /*
> + * the freelist can be empty if an interrupt triggered disconnect()
> + * and reconfigured the gadget (shutting down this queue) after the
> + * network stack decided to xmit but before we got the spinlock.
> + */
> + if (list_empty(&dev->tx_reqs)) {
> + spin_unlock_irqrestore(&dev->req_lock, flags);
> + return 1;
> + }
> +
> req = container_of (dev->tx_reqs.next, struct usb_request, list);
> list_del (&req->list);
> +
> + /* temporarily stop TX queue when the freelist empties */
> if (list_empty (&dev->tx_reqs))
> netif_stop_queue (net);
> spin_unlock_irqrestore(&dev->req_lock, flags);
>
>