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

[Takashi Iwai <tiwai@xxxxxxx>]

On Mon, 17 Feb 2025 19:38:31 +0100,
John Keeping wrote:
> 
> On Mon, Feb 17, 2025 at 06:06:16PM +0100, Takashi Iwai wrote:
> > 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.
> 
> Yes, this patch fixes it and is
> 
> Tested-by: John Keeping <jkeeping@xxxxxxxxxxxxxxxxx>

Thank you for quick testing!

Looking at the code again, I think the suggested fix isn't right.
It still allows some pending work accessing the freed object.

Could you test the following one-liner instead?


Takashi

-- 8< --
--- a/sound/usb/midi.c
+++ b/sound/usb/midi.c
@@ -1145,7 +1145,7 @@ static int snd_usbmidi_output_close(struct snd_rawmidi_substream *substream)
 {
 	struct usbmidi_out_port *port = substream->runtime->private_data;
 
-	cancel_work_sync(&port->ep->work);
+	flush_work(&port->ep->work);
 	return substream_open(substream, 0, 0);
 }