releases/4.19.9/alsa-pcm-fix-starvation-on-down_write_nonblock.patch

   1 From b888a5f713e4d17faaaff24316585a4eb07f35b7 Mon Sep 17 00:00:00 2001
   2 From: Chanho Min <chanho.min@lge.com>
   3 Date: Mon, 26 Nov 2018 14:36:37 +0900
   4 Subject: ALSA: pcm: Fix starvation on down_write_nonblock()
   5
   6 From: Chanho Min <chanho.min@lge.com>
   7
   8 commit b888a5f713e4d17faaaff24316585a4eb07f35b7 upstream.
   9
  10 Commit 67ec1072b053 ("ALSA: pcm: Fix rwsem deadlock for non-atomic PCM
  11 stream") fixes deadlock for non-atomic PCM stream. But, This patch
  12 causes antother stuck.
  13 If writer is RT thread and reader is a normal thread, the reader
  14 thread will be difficult to get scheduled. It may not give chance to
  15 release readlocks and writer gets stuck for a long time if they are
  16 pinned to single cpu.
  17
  18 The deadlock described in the previous commit is because the linux
  19 rwsem queues like a FIFO. So, we might need non-FIFO writelock, not
  20 non-block one.
  21
  22 My suggestion is that the writer gives reader a chance to be scheduled
  23 by using the minimum msleep() instaed of spinning without blocking by
  24 writer. Also, The *_nonblock may be changed to *_nonfifo appropriately
  25 to this concept.
  26 In terms of performance, when trylock is failed, this minimum periodic
  27 msleep will have the same performance as the tick-based
  28 schedule()/wake_up_q().
  29
  30 [ Although this has a fairly high performance penalty, the relevant
  31   code path became already rare due to the previous commit ("ALSA:
  32   pcm: Call snd_pcm_unlink() conditionally at closing").  That is, now
  33   this unconditional msleep appears only when using linked streams,
  34   and this must be a rare case.  So we accept this as a quick
  35   workaround until finding a more suitable one -- tiwai ]
  36
  37 Fixes: 67ec1072b053 ("ALSA: pcm: Fix rwsem deadlock for non-atomic PCM stream")
  38 Suggested-by: Wonmin Jung <wonmin.jung@lge.com>
  39 Signed-off-by: Chanho Min <chanho.min@lge.com>
  40 Cc: <stable@vger.kernel.org>
  41 Signed-off-by: Takashi Iwai <tiwai@suse.de>
  42 Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
  43
  44 ---
  45  sound/core/pcm_native.c |   11 ++++++-----
  46  1 file changed, 6 insertions(+), 5 deletions(-)
  47
  48 --- a/sound/core/pcm_native.c
  49 +++ b/sound/core/pcm_native.c
  50 @@ -36,6 +36,7 @@
  51  #include <sound/timer.h>
  52  #include <sound/minors.h>
  53  #include <linux/uio.h>
  54 +#include <linux/delay.h>
  55
  56  #include "pcm_local.h"
  57
  58 @@ -91,12 +92,12 @@ static DECLARE_RWSEM(snd_pcm_link_rwsem)
  59   * and this may lead to a deadlock when the code path takes read sem
  60   * twice (e.g. one in snd_pcm_action_nonatomic() and another in
  61   * snd_pcm_stream_lock()).  As a (suboptimal) workaround, let writer to
  62 - * spin until it gets the lock.
  63 + * sleep until all the readers are completed without blocking by writer.
  64   */
  65 -static inline void down_write_nonblock(struct rw_semaphore *lock)
  66 +static inline void down_write_nonfifo(struct rw_semaphore *lock)
  67  {
  68         while (!down_write_trylock(lock))
  69 -               cond_resched();
  70 +               msleep(1);
  71  }
  72
  73  #define PCM_LOCK_DEFAULT       0
  74 @@ -1967,7 +1968,7 @@ static int snd_pcm_link(struct snd_pcm_s
  75                 res = -ENOMEM;
  76                 goto _nolock;
  77         }
  78 -       down_write_nonblock(&snd_pcm_link_rwsem);
  79 +       down_write_nonfifo(&snd_pcm_link_rwsem);
  80         write_lock_irq(&snd_pcm_link_rwlock);
  81         if (substream->runtime->status->state == SNDRV_PCM_STATE_OPEN ||
  82             substream->runtime->status->state != substream1->runtime->status->state ||
  83 @@ -2014,7 +2015,7 @@ static int snd_pcm_unlink(struct snd_pcm
  84         struct snd_pcm_substream *s;
  85         int res = 0;
  86
  87 -       down_write_nonblock(&snd_pcm_link_rwsem);
  88 +       down_write_nonfifo(&snd_pcm_link_rwsem);
  89         write_lock_irq(&snd_pcm_link_rwlock);
  90         if (!snd_pcm_stream_linked(substream)) {
  91                 res = -EALREADY;