ALSA: usb-audio: Fix possible race at sync of urb completions

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.

Link: https://lore.kernel.org/r/20210929080844.11583-3-tiwai@suse.de
Signed-off-by: Takashi Iwai <tiwai@suse.de>

diff --git a/sound/usb/card.h b/sound/usb/card.h
index 3329ce7..746a765 100644 (file)
--- 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 29c4865..0624156 100644 (file)
--- 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);
+                       atomic_dec(&ep->submitted_urbs); /* decrement at last */
                        return;
                }
 
@@ -513,6 +515,7 @@ static void snd_complete_urb(struct urb *urb)
 
 exit_clear:
        clear_bit(ctx->index, &ep->active_mask);
+       atomic_dec(&ep->submitted_urbs);
 }
 
 /*
@@ -596,6 +599,7 @@ int snd_usb_add_endpoint(struct snd_usb_audio *chip, int ep_num, int type)
        ep->type = type;
        ep->ep_num = ep_num;
        INIT_LIST_HEAD(&ep->ready_playback_urbs);
+       atomic_set(&ep->submitted_urbs, 0);
 
        is_playback = ((ep_num & USB_ENDPOINT_DIR_MASK) == USB_DIR_OUT);
        ep_num &= USB_ENDPOINT_NUMBER_MASK;
@@ -861,7 +865,7 @@ static int wait_clear_urbs(struct snd_usb_endpoint *ep)
                return 0;
 
        do {
-               alive = bitmap_weight(&ep->active_mask, ep->nurbs);
+               alive = atomic_read(&ep->submitted_urbs);
                if (!alive)
                        break;
 
@@ -1441,6 +1445,7 @@ int snd_usb_endpoint_start(struct snd_usb_endpoint *ep)
                        goto __error;
                }
                set_bit(i, &ep->active_mask);
+               atomic_inc(&ep->submitted_urbs);
        }
 
        usb_audio_dbg(ep->chip, "%d URBs submitted for EP 0x%x\n",