ALSA: usb-audio: Use atomic_t for endpoint use_count

The endpoint objects may be started/stopped concurrently by different
substreams in the case of implicit feedback mode, while the current
code handles the reference counter without any protection.

This patch changes the refcount to atomic_t for avoiding the
inconsistency.  We need no reference_t here as the refcount goes only
up to 2.

Also the name "use_count" is renamed to "running" since this is about
actually the running status, not the open refcount.

Tested-by: Keith Milner <kamilner@superlative.org>
Tested-by: Dylan Robinson <dylan_robinson@motu.com>
Link: https://lore.kernel.org/r/20201123085347.19667-29-tiwai@suse.de
Signed-off-by: Takashi Iwai <tiwai@suse.de>

diff --git a/sound/usb/card.h b/sound/usb/card.h
index 53f0ce6..f58c376 100644 (file)
--- a/sound/usb/card.h
+++ b/sound/usb/card.h
@@ -60,7 +60,7 @@ struct snd_usb_endpoint {
        struct snd_usb_audio *chip;
 
        int opened;             /* open refcount; protect with chip->mutex */
-       int use_count;
+       atomic_t running;       /* running status */
        int ep_num;             /* the referenced endpoint number */
        int type;               /* SND_USB_ENDPOINT_TYPE_* */
        unsigned long flags;

diff --git a/sound/usb/endpoint.c b/sound/usb/endpoint.c
index 49fb934..4d733b2 100644 (file)
--- a/sound/usb/endpoint.c
+++ b/sound/usb/endpoint.c
@@ -1234,7 +1234,7 @@ unlock:
  *
  * @ep: the endpoint to start
  *
- * A call to this function will increment the use count of the endpoint.
+ * A call to this function will increment the running count of the endpoint.
  * In case it is not already running, the URBs for this endpoint will be
  * submitted. Otherwise, this function does nothing.
  *
@@ -1253,11 +1253,12 @@ int snd_usb_endpoint_start(struct snd_usb_endpoint *ep)
        if (ep->sync_master)
                WRITE_ONCE(ep->sync_master->sync_slave, ep);
 
-       usb_audio_dbg(ep->chip, "Starting %s EP 0x%x (count %d)\n",
-                     ep_type_name(ep->type), ep->ep_num, ep->use_count);
+       usb_audio_dbg(ep->chip, "Starting %s EP 0x%x (running %d)\n",
+                     ep_type_name(ep->type), ep->ep_num,
+                     atomic_read(&ep->running));
 
        /* already running? */
-       if (++ep->use_count != 1)
+       if (atomic_inc_return(&ep->running) != 1)
                return 0;
 
        /* just to be sure */
@@ -1319,7 +1320,7 @@ __error:
        if (ep->sync_master)
                WRITE_ONCE(ep->sync_master->sync_slave, NULL);
        clear_bit(EP_FLAG_RUNNING, &ep->flags);
-       ep->use_count--;
+       atomic_dec(&ep->running);
        deactivate_urbs(ep, false);
        return -EPIPE;
 }
@@ -1329,7 +1330,7 @@ __error:
  *
  * @ep: the endpoint to stop (may be NULL)
  *
- * A call to this function will decrement the use count of the endpoint.
+ * A call to this function will decrement the running count of the endpoint.
  * In case the last user has requested the endpoint stop, the URBs will
  * actually be deactivated.
  *
@@ -1343,16 +1344,17 @@ void snd_usb_endpoint_stop(struct snd_usb_endpoint *ep)
        if (!ep)
                return;
 
-       usb_audio_dbg(ep->chip, "Stopping %s EP 0x%x (count %d)\n",
-                     ep_type_name(ep->type), ep->ep_num, ep->use_count);
+       usb_audio_dbg(ep->chip, "Stopping %s EP 0x%x (running %d)\n",
+                     ep_type_name(ep->type), ep->ep_num,
+                     atomic_read(&ep->running));
 
-       if (snd_BUG_ON(ep->use_count == 0))
+       if (snd_BUG_ON(!atomic_read(&ep->running)))
                return;
 
        if (ep->sync_master)
                WRITE_ONCE(ep->sync_master->sync_slave, NULL);
 
-       if (--ep->use_count == 0) {
+       if (!atomic_dec_return(&ep->running)) {
                deactivate_urbs(ep, false);
                set_bit(EP_FLAG_STOPPING, &ep->flags);
        }
@@ -1363,7 +1365,7 @@ void snd_usb_endpoint_stop(struct snd_usb_endpoint *ep)
  *
  * @ep: the endpoint to release
  *
- * This function does not care for the endpoint's use count but will tear
+ * This function does not care for the endpoint's running count but will tear
  * down all the streaming URBs immediately.
  */
 void snd_usb_endpoint_release(struct snd_usb_endpoint *ep)
@@ -1410,7 +1412,7 @@ static void snd_usb_handle_sync_urb(struct snd_usb_endpoint *ep,
         * will take care of them later.
         */
        if (snd_usb_endpoint_implicit_feedback_sink(ep) &&
-           ep->use_count != 0) {
+           atomic_read(&ep->running)) {
 
                /* implicit feedback case */
                int i, bytes = 0;