ALSA: usb-audio: Fix possible race at sync of urb completions

USB-audio driver tries to sync with the clear of all pending URBs in
wait_clear_urbs(), and it waits for all bits in active_mask getting
cleared.  This works fine for the normal operations, but when a stream
is managed in the implicit feedback mode, there is still a very thin
race window: namely, in snd_complete_usb(), the active_mask bit for
the current URB is once cleared before re-submitted in
queue_pending_output_urbs().  If wait_clear_urbs() is called during
that period, it may pass the test and go forward even though there may
be a still pending URB.

For covering it, this patch adds a new counter to each endpoint to
keep the number of in-flight URBs, and changes wait_clear_urbs()
checking this number instead.  The counter is decremented at the end
of URB complete, hence the reference is kept as long as the URB
complete is in process.

Link: https://lore.kernel.org/r/20210929080844.11583-3-tiwai@suse.deSigned-off-by: Takashi Iwai <tiwai@suse.de>

sound/usb/card.h

@@ -97,6 +97,7 @@ struct snd_usb_endpoint {
 	unsigned int nominal_queue_size; /* total buffer sizes in URBs */
 	unsigned long active_mask;	/* bitmask of active urbs */
 	unsigned long unlink_mask;	/* bitmask of unlinked urbs */
+	atomic_t submitted_urbs;	/* currently submitted urbs */
 	char *syncbuf;			/* sync buffer for all sync URBs */
 	dma_addr_t sync_dma;		/* DMA address of syncbuf */
 

sound/usb/endpoint.c

@@ -451,6 +451,7 @@ static void queue_pending_output_urbs(struct snd_usb_endpoint *ep)
 		}
 
 		set_bit(ctx->index, &ep->active_mask);
+		atomic_inc(&ep->submitted_urbs);
 	}
 }
 
@@ -488,6 +489,7 @@ static void snd_complete_urb(struct urb *urb)
 			clear_bit(ctx->index, &ep->active_mask);
 			spin_unlock_irqrestore(&ep->lock, flags);
 			queue_pending_output_urbs(ep);
+			atomic_dec(&ep->submitted_urbs); /* decrement at last */
 			return;
 		}
 
@@ -513,6 +515,7 @@ static void snd_complete_urb(struct urb *urb)
 
 exit_clear:
 	clear_bit(ctx->index, &ep->active_mask);
+	atomic_dec(&ep->submitted_urbs);
 }
 
 /*
@@ -596,6 +599,7 @@ int snd_usb_add_endpoint(struct snd_usb_audio *chip, int ep_num, int type)
 	ep->type = type;
 	ep->ep_num = ep_num;
 	INIT_LIST_HEAD(&ep->ready_playback_urbs);
+	atomic_set(&ep->submitted_urbs, 0);
 
 	is_playback = ((ep_num & USB_ENDPOINT_DIR_MASK) == USB_DIR_OUT);
 	ep_num &= USB_ENDPOINT_NUMBER_MASK;
@@ -861,7 +865,7 @@ static int wait_clear_urbs(struct snd_usb_endpoint *ep)
 		return 0;
 
 	do {
-		alive = bitmap_weight(&ep->active_mask, ep->nurbs);
+		alive = atomic_read(&ep->submitted_urbs);
 		if (!alive)
 			break;
 
@@ -1441,6 +1445,7 @@ int snd_usb_endpoint_start(struct snd_usb_endpoint *ep)
 			goto __error;
 		}
 		set_bit(i, &ep->active_mask);
+		atomic_inc(&ep->submitted_urbs);
 	}
 
 	usb_audio_dbg(ep->chip, "%d URBs submitted for EP 0x%x\n",