USB: gadget: cdc-acm deadlock fix

This fixes a deadlock appearing with some USB peripheral drivers
when running CDC ACM gadget code.

The newish (2.6.27) CDC ACM event notification mechanism sends
messages (IN to the host) which are short enough to fit in most
FIFOs.  That means that with some peripheral controller drivers
(evidently not the ones used to verify the notification code!!)
the completion callback can be issued before queue() returns.

The deadlock would come because the completion callback and the
event-issuing code shared a spinlock.  Fix is trivial:  drop
that lock while queueing the message.

Signed-off-by: David Brownell <[email protected]>
Cc: stable <[email protected]>
Signed-off-by: Greg Kroah-Hartman <[email protected]>

drivers/usb/gadget/f_acm.c

@@ -463,7 +463,11 @@ static int acm_cdc_notify(struct f_acm *acm, u8 type, u16 value,
 	notify->wLength = cpu_to_le16(length);
 	memcpy(buf, data, length);
 
+	/* ep_queue() can complete immediately if it fills the fifo... */
+	spin_unlock(&acm->lock);
 	status = usb_ep_queue(ep, req, GFP_ATOMIC);
+	spin_lock(&acm->lock);
+
 	if (status < 0) {
 		ERROR(acm->port.func.config->cdev,
 				"acm ttyGS%d can't notify serial state, %d\n",