Re: [PATCH 2/9] ALSA: usb-audio: Fix possible race at sync of urb completions

[Takashi Iwai <tiwai@suse.de>]

On Fri, 18 Apr 2025 12:35:32 +0200,
Hillf Danton wrote:
> 
> On Wed, 29 Sep 2021 10:08:37 +0200 Takashi Iwai wrote:
> > 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.
> > 
> > Signed-off-by: Takashi Iwai <tiwai@suse.de>
> > ---
> >  sound/usb/card.h     | 1 +
> >  sound/usb/endpoint.c | 7 ++++++-
> >  2 files changed, 7 insertions(+), 1 deletion(-)
> > 
> > diff --git a/sound/usb/card.h b/sound/usb/card.h
> > index 3329ce710cb9..746a765b2437 100644
> > --- a/sound/usb/card.h
> > +++ b/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 */
> >  
> > diff --git a/sound/usb/endpoint.c b/sound/usb/endpoint.c
> > index 29c4865966f5..06241568abf7 100644
> > --- a/sound/usb/endpoint.c
> > +++ b/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);
> 
> 			smp_mb();
> 
> > +			atomic_dec(&ep->submitted_urbs); /* decrement at last */
> 
> Does it match the comment to add a mb?

How...?  I don't understand your intention.


Takashi