cdc-acm: close race betrween suspend() and acm_softint

Suspend increments a counter, then kills the URBs,
then kills the scheduled work. The scheduled work, however,
may reschedule the URBs. Fix this by having the work
check the counter.
Signed-off-by: Oliver Neukum <oneukum@suse.com>
Cc: stable <stable@vger.kernel.org>
Tested-by: Jonas Karlsson <jonas.karlsson@actia.se>
Link: https://lore.kernel.org/r/20200415151358.32664-1-oneukum@suse.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/usb/class/cdc-acm.c

@@ -557,14 +557,14 @@ static void acm_softint(struct work_struct *work)
 	struct acm *acm = container_of(work, struct acm, work);
 
 	if (test_bit(EVENT_RX_STALL, &acm->flags)) {
-		if (!(usb_autopm_get_interface(acm->data))) {
+		smp_mb(); /* against acm_suspend() */
+		if (!acm->susp_count) {
 			for (i = 0; i < acm->rx_buflimit; i++)
 				usb_kill_urb(acm->read_urbs[i]);
 			usb_clear_halt(acm->dev, acm->in);
 			acm_submit_read_urbs(acm, GFP_KERNEL);
-			usb_autopm_put_interface(acm->data);
+			clear_bit(EVENT_RX_STALL, &acm->flags);
 		}
-		clear_bit(EVENT_RX_STALL, &acm->flags);
 	}
 
 	if (test_and_clear_bit(EVENT_TTY_WAKEUP, &acm->flags))