ALSA: usb-audio: Fix irq/process data synchronization

Commit 16200948d83 ("ALSA: usb-audio: Fix race at stopping the stream") was
incomplete causing another more severe kernel panic, so it got reverted.
This fixes both the original problem and its fallout kernel race/crash.

The original fix is to move the endpoint member NULL clearing logic inside
wait_clear_urbs() so the irq triggering the urb completion doesn't call
retire_capture/playback_urb() after the NULL clearing and generate a panic.

However this creates a new race between snd_usb_endpoint_start()'s call
to wait_clear_urbs() and the irq urb completion handler which again calls
retire_capture/playback_urb() leading to a new NULL dereference.

We keep the EP deactivation code in snd_usb_endpoint_start() because
removing it will break the EP reference counting (see [1] [2] for info),
however we don't need the "can_sleep" mechanism anymore because a new
function was introduced (snd_usb_endpoint_sync_pending_stop()) which
synchronizes pending stops and gets called inside the pcm prepare callback.

It also makes sense to remove can_sleep because it was also removed from
deactivate_urbs() signature in [3] so we benefit from more simplification.

[1] commit 015618b90 ("ALSA: snd-usb: Fix URB cancellation at stream start")
[2] commit e9ba389c5 ("ALSA: usb-audio: Fix scheduling-while-atomic bug in PCM capture stream")
[3] commit ccc1696d5 ("ALSA: usb-audio: simplify endpoint deactivation code")

Fixes: f8114f8583bb ("Revert "ALSA: usb-audio: Fix race at stopping the stream"")

Signed-off-by: Ioan-Adrian Ratiu <adi@adirat.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>

3 files changed, 13 insertions, 16 deletions

diff --git a/sound/usb/endpoint.c b/sound/usb/endpoint.c
index 15d1d5c63c3c..2f0ea70a998c 100644
--- a/sound/usb/endpoint.c
+++ b/sound/usb/endpoint.c
@@ -534,6 +534,11 @@ static int wait_clear_urbs(struct snd_usb_endpoint *ep)
                         alive, ep->ep_num);
         clear_bit(EP_FLAG_STOPPING, &ep->flags);
 
+        ep->data_subs = NULL;
+        ep->sync_slave = NULL;
+        ep->retire_data_urb = NULL;
+        ep->prepare_data_urb = NULL;
+
         return 0;
 }
 
@@ -912,9 +917,7 @@ int snd_usb_endpoint_set_params(struct snd_usb_endpoint *ep,
 /**
  * snd_usb_endpoint_start: start an snd_usb_endpoint
  *
- * @ep:         the endpoint to start
- * @can_sleep:  flag indicating whether the operation is executed in
- *              non-atomic context
+ * @ep: the endpoint to start
  *
  * A call to this function will increment the use count of the endpoint.
  * In case it is not already running, the URBs for this endpoint will be
@@ -924,7 +927,7 @@ int snd_usb_endpoint_set_params(struct snd_usb_endpoint *ep,
  *
  * Returns an error if the URB submission failed, 0 in all other cases.
  */
-int snd_usb_endpoint_start(struct snd_usb_endpoint *ep, bool can_sleep)
+int snd_usb_endpoint_start(struct snd_usb_endpoint *ep)
 {
         int err;
         unsigned int i;
@@ -938,8 +941,6 @@ int snd_usb_endpoint_start(struct snd_usb_endpoint *ep, bool can_sleep)
 
         /* just to be sure */
         deactivate_urbs(ep, false);
-        if (can_sleep)
-                wait_clear_urbs(ep);
 
         ep->active_mask = 0;
         ep->unlink_mask = 0;
@@ -1020,10 +1021,6 @@ void snd_usb_endpoint_stop(struct snd_usb_endpoint *ep)
 
         if (--ep->use_count == 0) {
                 deactivate_urbs(ep, false);
-                ep->data_subs = NULL;
-                ep->sync_slave = NULL;
-                ep->retire_data_urb = NULL;
-                ep->prepare_data_urb = NULL;
                 set_bit(EP_FLAG_STOPPING, &ep->flags);
         }
 }
diff --git a/sound/usb/endpoint.h b/sound/usb/endpoint.h
index 6428392d8f62..584f295d7c77 100644
--- a/sound/usb/endpoint.h
+++ b/sound/usb/endpoint.h
@@ -18,7 +18,7 @@ int snd_usb_endpoint_set_params(struct snd_usb_endpoint *ep,
                                 struct audioformat *fmt,
                                 struct snd_usb_endpoint *sync_ep);
 
-int  snd_usb_endpoint_start(struct snd_usb_endpoint *ep, bool can_sleep);
+int  snd_usb_endpoint_start(struct snd_usb_endpoint *ep);
 void snd_usb_endpoint_stop(struct snd_usb_endpoint *ep);
 void snd_usb_endpoint_sync_pending_stop(struct snd_usb_endpoint *ep);
 int  snd_usb_endpoint_activate(struct snd_usb_endpoint *ep);
diff --git a/sound/usb/pcm.c b/sound/usb/pcm.c
index 34c6d4f2c0b6..9aa5b1855481 100644
--- a/sound/usb/pcm.c
+++ b/sound/usb/pcm.c
@@ -218,7 +218,7 @@ int snd_usb_init_pitch(struct snd_usb_audio *chip, int iface,
         }
 }
 
-static int start_endpoints(struct snd_usb_substream *subs, bool can_sleep)
+static int start_endpoints(struct snd_usb_substream *subs)
 {
         int err;
 
@@ -231,7 +231,7 @@ static int start_endpoints(struct snd_usb_substream *subs, bool can_sleep)
                 dev_dbg(&subs->dev->dev, "Starting data EP @%p\n", ep);
 
                 ep->data_subs = subs;
-                err = snd_usb_endpoint_start(ep, can_sleep);
+                err = snd_usb_endpoint_start(ep);
                 if (err < 0) {
                         clear_bit(SUBSTREAM_FLAG_DATA_EP_STARTED, &subs->flags);
                         return err;
@@ -260,7 +260,7 @@ static int start_endpoints(struct snd_usb_substream *subs, bool can_sleep)
                 dev_dbg(&subs->dev->dev, "Starting sync EP @%p\n", ep);
 
                 ep->sync_slave = subs->data_endpoint;
-                err = snd_usb_endpoint_start(ep, can_sleep);
+                err = snd_usb_endpoint_start(ep);
                 if (err < 0) {
                         clear_bit(SUBSTREAM_FLAG_SYNC_EP_STARTED, &subs->flags);
                         return err;
@@ -850,7 +850,7 @@ static int snd_usb_pcm_prepare(struct snd_pcm_substream *substream)
         /* for playback, submit the URBs now; otherwise, the first hwptr_done
          * updates for all URBs would happen at the same time when starting */
         if (subs->direction == SNDRV_PCM_STREAM_PLAYBACK)
-                ret = start_endpoints(subs, true);
+                ret = start_endpoints(subs);
 
  unlock:
         snd_usb_unlock_shutdown(subs->stream->chip);
@@ -1666,7 +1666,7 @@ static int snd_usb_substream_capture_trigger(struct snd_pcm_substream *substream
 
         switch (cmd) {
         case SNDRV_PCM_TRIGGER_START:
-                err = start_endpoints(subs, false);
+                err = start_endpoints(subs);
                 if (err < 0)
                         return err;