• Chanho Min's avatar
    ALSA: pcm: Fix starvation on down_write_nonblock() · b888a5f7
    Chanho Min authored
    Commit 67ec1072 ("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().
    
    [ Although this has a fairly high performance penalty, the relevant
      code path became already rare due to the previous commit ("ALSA:
      pcm: Call snd_pcm_unlink() conditionally at closing").  That is, now
      this unconditional msleep appears only when using linked streams,
      and this must be a rare case.  So we accept this as a quick
      workaround until finding a more suitable one -- tiwai ]
    
    Fixes: 67ec1072 ("ALSA: pcm: Fix rwsem deadlock for non-atomic PCM stream")
    Suggested-by: default avatarWonmin Jung <wonmin.jung@lge.com>
    Signed-off-by: default avatarChanho Min <chanho.min@lge.com>
    Cc: <stable@vger.kernel.org>
    Signed-off-by: default avatarTakashi Iwai <tiwai@suse.de>
    b888a5f7
pcm_native.c 101 KB