[ALSA] usb-audio: properly lock hwptr_done accesses

USB generic driver
Take the substream lock when reading hwptr_done to avoid a race
condition with the updates in the URB callbacks.

Signed-off-by: Clemens Ladisch <clemens@ladisch.de>

1 files changed, 8 insertions, 2 deletions

diff --git a/sound/usb/usbaudio.c b/sound/usb/usbaudio.c
index 3f7930c0b616..8d4a085f642a 100644
--- a/sound/usb/usbaudio.c
+++ b/sound/usb/usbaudio.c
@@ -815,8 +815,14 @@ static int wait_clear_urbs(snd_usb_substream_t *subs)
  */
 static snd_pcm_uframes_t snd_usb_pcm_pointer(snd_pcm_substream_t *substream)
 {
-        snd_usb_substream_t *subs = (snd_usb_substream_t *)substream->runtime->private_data;
-        return subs->hwptr_done;
+        snd_usb_substream_t *subs;
+        snd_pcm_uframes_t hwptr_done;
+        
+        subs = (snd_usb_substream_t *)substream->runtime->private_data;
+        spin_lock(&subs->lock);
+        hwptr_done = subs->hwptr_done;
+        spin_unlock(&subs->lock);
+        return hwptr_done;
 }