Re: [PATCH] usb: fix a task hung in snd_card_free

[Takashi Iwai <tiwai@suse.de>]

On Wed, 13 Nov 2024 02:48:49 +0100,
Edward Adam Davis wrote:
> 
> On Tue, 12 Nov 2024 17:04:04 +0100, Takashi Iwai wrote:
> > On Wed, 06 Nov 2024 03:15:49 +0100,
> > Edward Adam Davis wrote:
> > >
> > > task 1: snd ctrl will add card_dev ref count and can't call close to dec it,
> > >         it is blocked waiting for task 2 to release the USB dev lock.
> > >
> > > task 2: usb dev lock has been locked by hung task (here is usb_disconnect),
> > >         it is hung waiting for task 1 to exit and release card_dev.
> > >
> > > Adjust the USB lock acquisition method to non-blocking in ioctl to avoid
> > > hang when the USB connection is closed.
> > 
> > I'm afraid that this change would break things too badly.
> > i.e. changing the blocking behavior to non-blocking is no-go.
> > 
> > > Reported-and-tested-by: syzbot+73582d08864d8268b6fd@syzkaller.appspotmail.com
> > > Closes: https://syzkaller.appspot.com/bug?extid=73582d08864d8268b6fd
> > 
> > This particular syzkaller entry can be fixed rather by replacing
> > snd_card_free() in snd_usx2y_disconnect() with
> > snd_card_free_when_closed() like other USB audio drivers, something
> > like below.
> > 
> > Judging from the git log, it had been with snd_card_free_in_thread(),
> > but was switch to snd_card_free() around year 2005.  Meanwhile the
> > handling of async card release got improved, and it's very likely OK
> > to use snd_card_free_when_closed() there with the recent kernel.
> The snd_card instance will be released in snd_card_do_free().
> So, if snd_card_free_when_closed() is used to replace snd_card_free(), who will release the snd_card instance?

Via the release callback of the card device object, which is triggered
at the last close by refcounting.


Takashi

> 
> BR,
> Edward
> > 
> > 
> > thanks,
> > 
> > Takashi
> > 
> > -- 8< --
> > --- a/sound/usb/usx2y/usbusx2y.c
> > +++ b/sound/usb/usx2y/usbusx2y.c
> > @@ -422,7 +422,7 @@ static void snd_usx2y_disconnect(struct usb_interface *intf)
> >  	}
> >  	if (usx2y->us428ctls_sharedmem)
> >  		wake_up(&usx2y->us428ctls_wait_queue_head);
> > -	snd_card_free(card);
> > +	snd_card_free_when_closed(card);
> >  }
> > 
> >  static int snd_usx2y_probe(struct usb_interface *intf,
> 
>