[PATCH] ALSA: usb-audio: fix recursive lock of PCM substream when starting playback PCM substream

[Takashi Sakamoto]

A commit 9ce650a75a3b ("ALSA: usb-audio: Reduce latency at playback start")
unfortunately introduced the call of snd_pcm_period_elapsed() under acquired
lock of PCM substream. This causes recursive lock and results in dead-lock.

->ioctl(2)
  (sound/core/pcm_native.c)
  ->snd_pcm_stream_lock_irqsave() <-
    ...
    ->struct snd_pcm_ops.trigger()
    (sound/usb/pcm.c)
    = snd_usb_substream_playback_trigger()
      ->start_endpoints()
        (sound/usb/endpoint.c)
        ->snd_usb_endpoint_start()
          ->prepare_outbound_urb()
            ->struct snd_usb_endpoint.prepare_data_urb()
            (sound/usb/pcm.c)
            = prepare_playback_urb()
              (sound/core/pcm_lib.c)
              ->snd_pcm_period_elapsed()
                (sound/core/pcm_native.c)
                ->snd_pcm_stream_lock_irqsave() <-

This commit fixes the issue to use newly added function;
snd_pcm_period_elapsed_under_stream_lock() with condition to check running
context.

Reported-by: Hector Martin <marcan@xxxxxxxxx>
Fixes: 9ce650a75a3b ("ALSA: usb-audio: Reduce latency at playback start")
Signed-off-by: Takashi Sakamoto <o-takashi@xxxxxxxxxxxxx>
---
 sound/usb/pcm.c | 12 ++++++++++--
 1 file changed, 10 insertions(+), 2 deletions(-)

diff --git a/sound/usb/pcm.c b/sound/usb/pcm.c
index c66831ee15f9..235070f0236a 100644
--- a/sound/usb/pcm.c
+++ b/sound/usb/pcm.c
@@ -1395,8 +1395,16 @@ static void prepare_playback_urb(struct snd_usb_substream *subs,
 
 	spin_unlock_irqrestore(&subs->lock, flags);
 	urb->transfer_buffer_length = bytes;
-	if (period_elapsed)
-		snd_pcm_period_elapsed(subs->pcm_substream);
+	if (period_elapsed) {
+		// The callback of struct snd_pcm_ops.trigger with SNDRV_PCM_TRIGGER_START command
+		// can reach here, under acquired lock of PCM substream. To avoid dead-lock, check
+		// current context and call corresponding function.
+		if (in_softirq()) {
+			snd_pcm_period_elapsed(subs->pcm_substream);
+		} else {
+			snd_pcm_period_elapsed_under_stream_lock(subs->pcm_substream);
+		}
+	}
 }
 
 /*
-- 
2.30.2

======== 8< --------

Thanks

Takashi (not subsystem maintainer) Sakamoto