ALSA: usb-audio: More strict state change in EP

The endpoint management has bit flags to indicate the current state,
and we're dealing two things: the running bit and the stopping bit.
There is a thin window in transition from the running to the stopping
in stop_urbs(), and as long as the bit flags are used, it's difficult
to plug.

This patch modifies the state management code to use the atomic int
and follow the explicit three states, STOPPED, RUNNING and STOPPING.
The state change is done via atomic_cmpxhg() for avoiding possible
races, and check the state change more strictly.  The unexpected state
change is now handled as an error.

Fixes: d0f09d1e4a88 ("ALSA: usb-audio: Refactoring endpoint URB deactivation")
Cc: <stable@vger.kernel.org>
Link: https://lore.kernel.org/r/20210206203052.15606-3-tiwai@suse.de
Signed-off-by: Takashi Iwai <tiwai@suse.de>

2 files changed, 29 insertions, 15 deletions

diff --git a/sound/usb/card.h b/sound/usb/card.h
index 37091b117614..a741e7da83a2 100644
--- a/sound/usb/card.h
+++ b/sound/usb/card.h
@@ -71,7 +71,7 @@ struct snd_usb_endpoint {
 	unsigned char altsetting;	/* corresponding alternate setting */
 	unsigned char ep_idx;		/* endpoint array index */
 
-	unsigned long flags;	/* running bit flags */
+	atomic_t state;		/* running state */
 
 	void (*prepare_data_urb) (struct snd_usb_substream *subs,
 				  struct urb *urb);
diff --git a/sound/usb/endpoint.c b/sound/usb/endpoint.c
index e102c024c21f..4390075b2c6f 100644
--- a/sound/usb/endpoint.c
+++ b/sound/usb/endpoint.c
@@ -21,8 +21,11 @@
 #include "clock.h"
 #include "quirks.h"
 
-#define EP_FLAG_RUNNING		1
-#define EP_FLAG_STOPPING	2
+enum {
+	EP_STATE_STOPPED,
+	EP_STATE_RUNNING,
+	EP_STATE_STOPPING,
+};
 
 /* interface refcounting */
 struct snd_usb_iface_ref {
@@ -115,6 +118,16 @@ static const char *usb_error_string(int err)
 	}
 }
 
+static inline bool ep_state_running(struct snd_usb_endpoint *ep)
+{
+	return atomic_read(&ep->state) == EP_STATE_RUNNING;
+}
+
+static inline bool ep_state_update(struct snd_usb_endpoint *ep, int old, int new)
+{
+	return atomic_cmpxchg(&ep->state, old, new) == old;
+}
+
 /**
  * snd_usb_endpoint_implicit_feedback_sink: Report endpoint usage type
  *
@@ -393,7 +406,7 @@ next_packet_fifo_dequeue(struct snd_usb_endpoint *ep)
  */
 static void queue_pending_output_urbs(struct snd_usb_endpoint *ep)
 {
-	while (test_bit(EP_FLAG_RUNNING, &ep->flags)) {
+	while (ep_state_running(ep)) {
 
 		unsigned long flags;
 		struct snd_usb_packet_info *packet;
@@ -454,13 +467,13 @@ static void snd_complete_urb(struct urb *urb)
 	if (unlikely(atomic_read(&ep->chip->shutdown)))
 		goto exit_clear;
 
-	if (unlikely(!test_bit(EP_FLAG_RUNNING, &ep->flags)))
+	if (unlikely(!ep_state_running(ep)))
 		goto exit_clear;
 
 	if (usb_pipeout(ep->pipe)) {
 		retire_outbound_urb(ep, ctx);
 		/* can be stopped during retire callback */
-		if (unlikely(!test_bit(EP_FLAG_RUNNING, &ep->flags)))
+		if (unlikely(!ep_state_running(ep)))
 			goto exit_clear;
 
 		if (snd_usb_endpoint_implicit_feedback_sink(ep)) {
@@ -474,12 +487,12 @@ static void snd_complete_urb(struct urb *urb)
 
 		prepare_outbound_urb(ep, ctx);
 		/* can be stopped during prepare callback */
-		if (unlikely(!test_bit(EP_FLAG_RUNNING, &ep->flags)))
+		if (unlikely(!ep_state_running(ep)))
 			goto exit_clear;
 	} else {
 		retire_inbound_urb(ep, ctx);
 		/* can be stopped during retire callback */
-		if (unlikely(!test_bit(EP_FLAG_RUNNING, &ep->flags)))
+		if (unlikely(!ep_state_running(ep)))
 			goto exit_clear;
 
 		prepare_inbound_urb(ep, ctx);
@@ -835,7 +848,7 @@ static int wait_clear_urbs(struct snd_usb_endpoint *ep)
 	unsigned long end_time = jiffies + msecs_to_jiffies(1000);
 	int alive;
 
-	if (!test_bit(EP_FLAG_STOPPING, &ep->flags))
+	if (atomic_read(&ep->state) != EP_STATE_STOPPING)
 		return 0;
 
 	do {
@@ -850,10 +863,11 @@ static int wait_clear_urbs(struct snd_usb_endpoint *ep)
 		usb_audio_err(ep->chip,
 			"timeout: still %d active urbs on EP #%x\n",
 			alive, ep->ep_num);
-	clear_bit(EP_FLAG_STOPPING, &ep->flags);
 
-	ep->sync_sink = NULL;
-	snd_usb_endpoint_set_callback(ep, NULL, NULL, NULL);
+	if (ep_state_update(ep, EP_STATE_STOPPING, EP_STATE_STOPPED)) {
+		ep->sync_sink = NULL;
+		snd_usb_endpoint_set_callback(ep, NULL, NULL, NULL);
+	}
 
 	return 0;
 }
@@ -882,10 +896,9 @@ static int stop_urbs(struct snd_usb_endpoint *ep, bool force)
 	if (!force && atomic_read(&ep->running))
 		return -EBUSY;
 
-	if (!test_and_clear_bit(EP_FLAG_RUNNING, &ep->flags))
+	if (!ep_state_update(ep, EP_STATE_RUNNING, EP_STATE_STOPPING))
 		return 0;
 
-	set_bit(EP_FLAG_STOPPING, &ep->flags);
 	INIT_LIST_HEAD(&ep->ready_playback_urbs);
 	ep->next_packet_head = 0;
 	ep->next_packet_queued = 0;
@@ -1362,7 +1375,8 @@ int snd_usb_endpoint_start(struct snd_usb_endpoint *ep)
 	 * from that context.
 	 */
 
-	set_bit(EP_FLAG_RUNNING, &ep->flags);
+	if (!ep_state_update(ep, EP_STATE_STOPPED, EP_STATE_RUNNING))
+		goto __error;
 
 	if (snd_usb_endpoint_implicit_feedback_sink(ep)) {
 		for (i = 0; i < ep->nurbs; i++) {