Re: [PATCH] ALSA: pcm: Fix starvation on down_write_nonblock()

[Takashi Iwai]

On Mon, 26 Nov 2018 06:36:37 +0100,
Chanho Min wrote:
> 
> Commit 67ec1072b053 ("ALSA: pcm: Fix rwsem deadlock for non-atomic PCM stream")
> fixes deadlock for non-atomic PCM stream. But, This patch causes antother stuck.
> If writer is RT thread and reader is a normal thread, the reader thread will
> be difficult to get scheduled. It may not give chance to release readlocks
> and writer gets stuck for a long time if they are pinned to single cpu.
> 
> The deadlock described in the previous commit is because the linux rwsem
> queues like a FIFO. So, we might need non-FIFO writelock, not non-block one.
> 
> My suggestion is that the writer gives reader a chance to be scheduled by using
> the minimum msleep() instaed of spinning without blocking by writer. Also,
> The *_nonblock may be changed to *_nonfifo appropriately to this concept.
> In terms of performance, when trylock is failed, this minimum periodic msleep
> will have the same performance as the tick-based schedule()/wake_up_q().
> 
> Suggested-by: Wonmin Jung <wonmin.jung@xxxxxxx>
> Signed-off-by: Chanho Min <chanho.min@xxxxxxx>

Hrm, converting unconditionally with msleep() looks too drastic.

I guess you've hit this while not explicitly using the linked PCM
streams, i.e. in the call of snd_pcm_unlink() at close, right?

Then this can be worked around by checking the link before calling
it.  Could you check the patch below?


thanks,

Takashi

--- a/sound/core/pcm_native.c
+++ b/sound/core/pcm_native.c
@@ -2369,7 +2369,8 @@ int snd_pcm_hw_constraints_complete(struct snd_pcm_substream *substream)
 
 static void pcm_release_private(struct snd_pcm_substream *substream)
 {
-	snd_pcm_unlink(substream);
+	if (snd_pcm_stream_linked(substream))
+		snd_pcm_unlink(substream);
 }
 
 void snd_pcm_release_substream(struct snd_pcm_substream *substream)