ALSA: usb - Fix Oops after usb-midi disconnection

usb-midi causes sometimes Oops at snd_usbmidi_output_drain() after
disconnection.  This is due to the access to the endpoints which have
been already released at disconnection while the files are still alive.

This patch fixes the problem by checking disconnection state at
snd_usbmidi_output_drain() and by releasing urbs but keeping the
endpoint instances until really all freed.
Tested-by: Tvrtko Ursulin <tvrtko@ursulin.net>
Cc: <stable@kernel.org>
Signed-off-by: Takashi Iwai <tiwai@suse.de>

sound/usb/usbmidi.c

@@ -986,6 +986,8 @@ static void snd_usbmidi_output_drain(struct snd_rawmidi_substream *substream)
 	DEFINE_WAIT(wait);
 	long timeout = msecs_to_jiffies(50);
 
+	if (ep->umidi->disconnected)
+		return;
 	/*
 	 * The substream buffer is empty, but some data might still be in the
 	 * currently active URBs, so we have to wait for those to complete.
@@ -1123,14 +1125,21 @@ static int snd_usbmidi_in_endpoint_create(struct snd_usb_midi* umidi,
  * Frees an output endpoint.
  * May be called when ep hasn't been initialized completely.
  */
-static void snd_usbmidi_out_endpoint_delete(struct snd_usb_midi_out_endpoint* ep)
+static void snd_usbmidi_out_endpoint_clear(struct snd_usb_midi_out_endpoint *ep)
 {
 	unsigned int i;
 
 	for (i = 0; i < OUTPUT_URBS; ++i)
-		if (ep->urbs[i].urb)
+		if (ep->urbs[i].urb) {
 			free_urb_and_buffer(ep->umidi, ep->urbs[i].urb,
 					    ep->max_transfer);
+			ep->urbs[i].urb = NULL;
+		}
+}
+
+static void snd_usbmidi_out_endpoint_delete(struct snd_usb_midi_out_endpoint *ep)
+{
+	snd_usbmidi_out_endpoint_clear(ep);
 	kfree(ep);
 }
 
@@ -1262,15 +1271,18 @@ void snd_usbmidi_disconnect(struct list_head* p)
 				usb_kill_urb(ep->out->urbs[j].urb);
 			if (umidi->usb_protocol_ops->finish_out_endpoint)
 				umidi->usb_protocol_ops->finish_out_endpoint(ep->out);
+			ep->out->active_urbs = 0;
+			if (ep->out->drain_urbs) {
+				ep->out->drain_urbs = 0;
+				wake_up(&ep->out->drain_wait);
+			}
 		}
 		if (ep->in)
 			for (j = 0; j < INPUT_URBS; ++j)
 				usb_kill_urb(ep->in->urbs[j]);
 		/* free endpoints here; later call can result in Oops */
-		if (ep->out) {
-			snd_usbmidi_out_endpoint_delete(ep->out);
-			ep->out = NULL;
-		}
+		if (ep->out)
+			snd_usbmidi_out_endpoint_clear(ep->out);
 		if (ep->in) {
 			snd_usbmidi_in_endpoint_delete(ep->in);
 			ep->in = NULL;