Re: [PATCH] ALSA: usb-audio: Apply async workaround for Scarlett 2i4 2nd gen

[Takashi Iwai <tiwai@suse.de>]

On Thu, 23 Apr 2020 19:29:08 +0200,
Takashi Iwai wrote:
> 
> On Thu, 23 Apr 2020 18:57:34 +0200,
> Alexander Tsoy wrote:
> > 
> > And some further notes:
> > 
> > - I removed locking from snd_usb_endpoint_next_packet_size() and this
> > seems completely fixed an issue with large URBs I reported here:
> > 
> > https://bugzilla.kernel.org/show_bug.cgi?id=199327#c28
> > 
> > So playing at 96 kHz, driver packs 48 frames per URB and no more audio
> > discontinuities.
> 
> Hmm, that's weird.
> 
> If removing the lock from snd_usb_endpoint_next_packet_size() really
> fixes the problem, it implies the lock contention.  But as far as I
> see the code performed in this lock isn't conflicting so much.  The
> URB processing shouldn't happen in parallel for the same EP.

BTW, one potential racy code I found while looking at the code is the
list management in queue_pending_output_urbs().  The fix patch is
below.


Takashi

--- a/sound/usb/endpoint.c
+++ b/sound/usb/endpoint.c
@@ -321,17 +321,17 @@ static void queue_pending_output_urbs(struct snd_usb_endpoint *ep)
 			ep->next_packet_read_pos %= MAX_URBS;
 
 			/* take URB out of FIFO */
-			if (!list_empty(&ep->ready_playback_urbs))
+			if (!list_empty(&ep->ready_playback_urbs)) {
 				ctx = list_first_entry(&ep->ready_playback_urbs,
 					       struct snd_urb_ctx, ready_list);
+				list_del_init(&ctx->ready_list);
+			}
 		}
 		spin_unlock_irqrestore(&ep->lock, flags);
 
 		if (ctx == NULL)
 			return;
 
-		list_del_init(&ctx->ready_list);
-
 		/* copy over the length information */
 		for (i = 0; i < packet->packets; i++)
 			ctx->packet_size[i] = packet->packet_size[i];