ALSA: usb-audio: Fix crash at re-preparing the PCM stream

There are bug reports of a crash with USB-audio devices when PCM
prepare is performed immediately after the stream is stopped via
trigger callback.  It turned out that the problem is that we don't
wait until all URBs are killed.

This patch adds a new function to synchronize the pending stop
operation on an endpoint, and calls in the prepare callback for
avoiding the crash above.

Bugzilla: https://bugzilla.kernel.org/show_bug.cgi?id=49181

Reported-and-tested-by: Artem S. Tashkinov <t.artem@lycos.com>
Cc: <stable@vger.kernel.org> [v3.6]
Signed-off-by: Takashi Iwai <tiwai@suse.de>

sound/usb/endpoint.c

@@ -35,6 +35,7 @@
 
 #define EP_FLAG_ACTIVATED	0
 #define EP_FLAG_RUNNING		1
+#define EP_FLAG_STOPPING	2
 
 /*
  * snd_usb_endpoint is a model that abstracts everything related to an
@@ -502,10 +503,20 @@ static int wait_clear_urbs(struct snd_usb_endpoint *ep)
 	if (alive)
 		snd_printk(KERN_ERR "timeout: still %d active urbs on EP #%x\n",
 					alive, ep->ep_num);
+	clear_bit(EP_FLAG_STOPPING, &ep->flags);
 
 	return 0;
 }
 
+/* sync the pending stop operation;
+ * this function itself doesn't trigger the stop operation
+ */
+void snd_usb_endpoint_sync_pending_stop(struct snd_usb_endpoint *ep)
+{
+	if (ep && test_bit(EP_FLAG_STOPPING, &ep->flags))
+		wait_clear_urbs(ep);
+}
+
 /*
  * unlink active urbs.
  */
@@ -918,6 +929,8 @@ void snd_usb_endpoint_stop(struct snd_usb_endpoint *ep,
 
 		if (wait)
 			wait_clear_urbs(ep);
+		else
+			set_bit(EP_FLAG_STOPPING, &ep->flags);
 	}
 }