Re: [BUG] ALSA: usb-audio: drain may fail with multi-port close race

[Takashi Iwai <tiwai@xxxxxxx>]

On Mon, 17 Feb 2025 12:16:46 +0100,
John Keeping wrote:
> 
> I'm seeing a bug where data sometimes fails to send on USB MIDI devices
> with multiple ports which seems to be a result of a race around closing
> ports introduced by commit 0125de38122f0 ("ALSA: usb-audio: Cancel
> pending work at closing a MIDI substream").
> 
> The scenario is essentially this program:
> 
> 	snd_rawmidi_t *port0, *port1;
> 	snd_rawmidi_open(NULL, &port0, "hw:0,0,0", 0);
> 	snd_rawmidi_open(NULL, &port1, "hw:0,0,1", 0);
> 
> 	snd_rawmidi_write(port0, data, len);
> 
> 	snd_rawmidi_close(port1);
> 	snd_rawmidi_close(port0);
> 
> What happens seems to be the following:
> 
> 	write(port0)
> 	`- snd_usbmidi_output_trigger
> 	   `- queue_work()
>         close(port1)
> 	`- snd_usbmidi_output_close
> 	   `- cancel_work_sync()	# Work has not yet started here
> 	close(port0)
> 	`- snd_rawmidi_drain_output
> 	   # Times out because nothing is processing outbound data!
> 
> The two ports interact like this because they are on the same endpoint,
> so should the work only be cancelled when the last endpoint is closed?

How about the following patch work?
It's a band-aid, but should suffice.  The callback is already
protected with rawmidi open_mutex.


thanks,

Takashi

-- 8< --
--- a/sound/usb/midi.c
+++ b/sound/usb/midi.c
@@ -1144,8 +1144,11 @@ static int snd_usbmidi_output_open(struct snd_rawmidi_substream *substream)
 static int snd_usbmidi_output_close(struct snd_rawmidi_substream *substream)
 {
 	struct usbmidi_out_port *port = substream->runtime->private_data;
+	struct snd_usb_midi *umidi = substream->rmidi->private_data;
 
-	cancel_work_sync(&port->ep->work);
+	/* cancel at the last close */
+	if (umidi->opened[0] == 1)
+		cancel_work_sync(&port->ep->work);
 	return substream_open(substream, 0, 0);
 }