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

[John Keeping <jkeeping@xxxxxxxxxxxxxxxxx>]

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?


Thanks,
John