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

[Takashi Iwai]

On Thu, 29 Nov 2018 00:48:20 +0100,
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