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

[Chanho Min]

> Chanho Min wrote:
> >
> > > > > 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.
> > >
> > > Right, but applying msleep(1) unconditionally is really bad.
> > >
> > > > > 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.
> > >
> > > Then I'm sure that my patch papers over.
> > Thanks, Plz let me know when the patch is merged.
> 
> I'm going to merge the patch below now.
> It slips from the pull request to Linus in today, but will be the next
> one for 4.20-rc6.
> 
> > > > But, If the linked PCM is enabled,  Can snd_pcm_unlink() be called?
> > > > This also seems to be a workaround.
> > >
> > > Yes, for the linked streams, something else is needed *in addition*.
> > >
> > > The original fix with busy loop also assumed that this code path (via
> > > snd_pcm_link() and snd_pcm_unlink()) is the rare occasion, and it
> didn't
> > > consider that it were called for regular use cases.  So the fix to
> make
> > > things just works for regular use cases without any artifact must be
> > > implemented in the first place.  The fix for the linked streams comes
> at
> > > next.  It might be like your msleep() change as a workaround, but in
> > > anyway it's far less urgency.
> >
> > msleep is worst, but If it is harmless, can I apply my patch to the
> private
> > tree
> > temporarily until your next fix comes?
> > We may use the linked streams in the near future. It makes our product
> > unstable.
> > It's urgency for us. How is your opinion?
> 
> I'll add your fix on top of mine for now.  The msleep() is applied
> only for linked streams, so it's not that bad any longer.
> 
> 
> thanks,
> 
> Takashi
> 
> -- 8< --
> From: Takashi Iwai <tiwai@xxxxxxx>
> Subject: [PATCH] ALSA: pcm: Call snd_pcm_unlink() conditionally at closing
> 
> Currently the PCM core calls snd_pcm_unlink() always unconditionally
> at closing a stream.  However, since snd_pcm_unlink() invokes the
> global rwsem down, the lock can be easily contended.  More badly, when
> a thread runs in a high priority RT-FIFO, it may stall at spinning.
> 
> Basically the call of snd_pcm_unlink() is required only for the linked
> streams that are already rare occasion.  For normal use cases, this
> code path is fairly superfluous.
> 
> As an optimization (and also as a workaround for the RT problem
> above in normal situations without linked streams), this patch adds a
> check before calling snd_pcm_unlink() and calls it only when needed.
> 
> Reported-by: Chanho Min <chanho.min@xxxxxxx>
> Cc: <stable@xxxxxxxxxxxxxxx>
> Signed-off-by: Takashi Iwai <tiwai@xxxxxxx>
> ---
>  sound/core/pcm_native.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/sound/core/pcm_native.c b/sound/core/pcm_native.c
> index 66c90f486af9..6afcc393113a 100644
> --- 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)
> --
> 2.19.1
Great, Many thanks for the fast response.

Chanho