[PATCH] ALSA: usb-audio: Fix recursive locking on XRUN

[John Keeping]

snd_usb_queue_pending_output_urbs() may be called from
snd_pcm_ops::ack() which means the PCM stream is locked.

For the normal case where the call back into the PCM core is via
prepare_output_urb() the "_under_stream_lock" variant of
snd_pcm_period_elapsed() is called, but when an error occurs and the
stream is stopped as XRUN then snd_pcm_xrun() tries to recursively lock
the stream which results in deadlock.

Follow the example of snd_pcm_period_elapsed() by adding
snd_pcm_xrun_under_stream_lock() and use this when the PCM substream
lock is already held.

Signed-off-by: John Keeping <john@xxxxxxxxxxxx>
---
 include/sound/pcm.h     |  1 +
 sound/core/pcm_native.c | 28 ++++++++++++++++++++++++----
 sound/usb/endpoint.c    | 18 +++++++++++-------
 3 files changed, 36 insertions(+), 11 deletions(-)

diff --git a/include/sound/pcm.h b/include/sound/pcm.h
index 27040b472a4f..98551907453a 100644
--- a/include/sound/pcm.h
+++ b/include/sound/pcm.h
@@ -571,6 +571,7 @@ int snd_pcm_status64(struct snd_pcm_substream *substream,
 int snd_pcm_start(struct snd_pcm_substream *substream);
 int snd_pcm_stop(struct snd_pcm_substream *substream, snd_pcm_state_t status);
 int snd_pcm_drain_done(struct snd_pcm_substream *substream);
+int snd_pcm_stop_xrun_under_stream_lock(struct snd_pcm_substream *substream);
 int snd_pcm_stop_xrun(struct snd_pcm_substream *substream);
 #ifdef CONFIG_PM
 int snd_pcm_suspend_all(struct snd_pcm *pcm);
diff --git a/sound/core/pcm_native.c b/sound/core/pcm_native.c
index 331380c2438b..617f5dc74df0 100644
--- a/sound/core/pcm_native.c
+++ b/sound/core/pcm_native.c
@@ -1559,24 +1559,44 @@ int snd_pcm_drain_done(struct snd_pcm_substream *substream)
 				     SNDRV_PCM_STATE_SETUP);
 }
 
+/**
+ * snd_pcm_stop_xrun_under_stream_lock - stop the running stream as XRUN under the lock of
+ * 					 the PCM substream.
+ * @substream: the PCM substream instance
+ *
+ * This stops the given running substream (and all linked substreams) as XRUN.
+ * This function assumes that the substream lock is already held.
+ *
+ * Return: Zero if successful, or a negative error core.
+ */
+int snd_pcm_stop_xrun_under_stream_lock(struct snd_pcm_substream *substream)
+{
+	if (substream->runtime && snd_pcm_running(substream))
+		__snd_pcm_xrun(substream);
+
+	return 0;
+}
+
 /**
  * snd_pcm_stop_xrun - stop the running streams as XRUN
  * @substream: the PCM substream instance
  *
+ * This function is similar to ``snd_pcm_stop_xrun_under_stream_lock()`` except that it
+ * acquires the substream lock itself.
+ *
  * This stops the given running substream (and all linked substreams) as XRUN.
- * Unlike snd_pcm_stop(), this function takes the substream lock by itself.
  *
  * Return: Zero if successful, or a negative error code.
  */
 int snd_pcm_stop_xrun(struct snd_pcm_substream *substream)
 {
 	unsigned long flags;
+	int ret;
 
 	snd_pcm_stream_lock_irqsave(substream, flags);
-	if (substream->runtime && snd_pcm_running(substream))
-		__snd_pcm_xrun(substream);
+	ret = snd_pcm_stop_xrun_under_stream_lock(substream);
 	snd_pcm_stream_unlock_irqrestore(substream, flags);
-	return 0;
+	return ret;
 }
 EXPORT_SYMBOL_GPL(snd_pcm_stop_xrun);
 
diff --git a/sound/usb/endpoint.c b/sound/usb/endpoint.c
index 1e0af1179ca8..83a6b6d41374 100644
--- a/sound/usb/endpoint.c
+++ b/sound/usb/endpoint.c
@@ -400,13 +400,17 @@ static int prepare_inbound_urb(struct snd_usb_endpoint *ep,
 }
 
 /* notify an error as XRUN to the assigned PCM data substream */
-static void notify_xrun(struct snd_usb_endpoint *ep)
+static void notify_xrun(struct snd_usb_endpoint *ep, bool in_stream_lock)
 {
 	struct snd_usb_substream *data_subs;
 
 	data_subs = READ_ONCE(ep->data_subs);
-	if (data_subs && data_subs->pcm_substream)
-		snd_pcm_stop_xrun(data_subs->pcm_substream);
+	if (data_subs && data_subs->pcm_substream) {
+		if (in_stream_lock)
+			snd_pcm_stop_xrun_under_stream_lock(data_subs->pcm_substream);
+		else
+			snd_pcm_stop_xrun(data_subs->pcm_substream);
+	}
 }
 
 static struct snd_usb_packet_info *
@@ -498,7 +502,7 @@ void snd_usb_queue_pending_output_urbs(struct snd_usb_endpoint *ep,
 			if (err == -EAGAIN)
 				push_back_to_ready_list(ep, ctx);
 			else
-				notify_xrun(ep);
+				notify_xrun(ep, in_stream_lock);
 			return;
 		}
 
@@ -507,7 +511,7 @@ void snd_usb_queue_pending_output_urbs(struct snd_usb_endpoint *ep,
 			usb_audio_err(ep->chip,
 				      "Unable to submit urb #%d: %d at %s\n",
 				      ctx->index, err, __func__);
-			notify_xrun(ep);
+			notify_xrun(ep, in_stream_lock);
 			return;
 		}
 
@@ -574,7 +578,7 @@ static void snd_complete_urb(struct urb *urb)
 		return;
 
 	usb_audio_err(ep->chip, "cannot submit urb (err = %d)\n", err);
-	notify_xrun(ep);
+	notify_xrun(ep, false);
 
 exit_clear:
 	clear_bit(ctx->index, &ep->active_mask);
@@ -1762,7 +1766,7 @@ static void snd_usb_handle_sync_urb(struct snd_usb_endpoint *ep,
 			usb_audio_err(ep->chip,
 				      "next package FIFO overflow EP 0x%x\n",
 				      ep->ep_num);
-			notify_xrun(ep);
+			notify_xrun(ep, false);
 			return;
 		}
 
-- 
2.40.0