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

[Chanho Min]

> 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.

Yes, it looks drastic. But, IMHO, I can't say busy-spin is not non-drastic.
To fix the root cause, We may need another rwsem that does not work as a
FIFO.

> 
> 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?

More testing is needed, but it seems to be fixed by your patch.
We may not use the linked PCM.
But, If the linked PCM is enabled,  Can snd_pcm_unlink() be called?
This also seems to be a workaround.

> 
> 
> 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)