ALSA: pcm: Revert capture stream behavior change in blocking mode

In the commit 62ba568f7aef ("ALSA: pcm: Return 0 when size <
start_threshold in capture"), we changed the behavior of
__snd_pcm_lib_xfer() to return immediately with 0 when a capture
stream has a high start_threshold.  This was intended to be a
correction of the behavior consistency and looked harmless, but this
was the culprit of the recent breakage reported by syzkaller, which
was fixed by the commit e190161f96b8 ("ALSA: pcm: Fix tight loop of
OSS capture stream").

At the time for the OSS fix, I didn't touch the behavior for ALSA
native API, as assuming that this behavior actually is good.  But this
turned out to be also broken actually for a similar deployment,
e.g. one thread goes to a write loop in blocking mode while another
thread controls the start/stop of the stream manually.

Overall, the original commit is harmful, and it brings less merit to
keep that behavior.  Let's revert it.

Fixes: 62ba568f7aef ("ALSA: pcm: Return 0 when size < start_threshold in capture")
Fixes: e190161f96b8 ("ALSA: pcm: Fix tight loop of OSS capture stream")
Cc: <stable@vger.kernel.org>
Signed-off-by: Takashi Iwai <tiwai@suse.de>

1 files changed, 4 insertions, 16 deletions

diff --git a/sound/core/pcm_lib.c b/sound/core/pcm_lib.c
index 6c99fa8ac5fa..6c0b30391ba9 100644
--- a/sound/core/pcm_lib.c
+++ b/sound/core/pcm_lib.c
@@ -2112,13 +2112,6 @@ int pcm_lib_apply_appl_ptr(struct snd_pcm_substream *substream,
         return 0;
 }
 
-/* allow waiting for a capture stream that hasn't been started */
-#if IS_ENABLED(CONFIG_SND_PCM_OSS)
-#define wait_capture_start(substream)   ((substream)->oss.oss)
-#else
-#define wait_capture_start(substream)   false
-#endif
-
 /* the common loop for read/write data */
 snd_pcm_sframes_t __snd_pcm_lib_xfer(struct snd_pcm_substream *substream,
                                      void *data, bool interleaved,
@@ -2184,16 +2177,11 @@ snd_pcm_sframes_t __snd_pcm_lib_xfer(struct snd_pcm_substream *substream,
                 snd_pcm_update_hw_ptr(substream);
 
         if (!is_playback &&
-            runtime->status->state == SNDRV_PCM_STATE_PREPARED) {
-                if (size >= runtime->start_threshold) {
-                        err = snd_pcm_start(substream);
-                        if (err < 0)
-                                goto _end_unlock;
-                } else if (!wait_capture_start(substream)) {
-                        /* nothing to do */
-                        err = 0;
+            runtime->status->state == SNDRV_PCM_STATE_PREPARED &&
+            size >= runtime->start_threshold) {
+                err = snd_pcm_start(substream);
+                if (err < 0)
                         goto _end_unlock;
-                }
         }
 
         avail = snd_pcm_avail(substream);