ALSA: seq_oss: fix waitqueue_active without memory barrier in snd-seq-oss
authorKosuke Tatsukawa <tatsu@ab.jp.nec.com>
Fri, 9 Oct 2015 00:35:54 +0000 (00:35 +0000)
committerTakashi Iwai <tiwai@suse.de>
Fri, 9 Oct 2015 07:45:52 +0000 (09:45 +0200)
commit694470273de29c2d3f3792856d4e748969294789
treed805ff974fde0d31db4f8504f52b265941f6de9f
parent70b4891cc889d359d3b1f3b8ec6f10c5800ecc70
ALSA: seq_oss: fix waitqueue_active without memory barrier in snd-seq-oss

snd_seq_oss_readq_put_event() seems to be missing a memory barrier which
might cause the waker to not notice the waiter and miss sending a
wake_up as in the following figure.

    snd_seq_oss_readq_put_event     snd_seq_oss_readq_wait
------------------------------------------------------------------------
/* wait_event_interruptible_timeout */
 /* __wait_event_interruptible_timeout */
  /* ___wait_event */
  for (;;) {  prepare_to_wait_event(&wq, &__wait,
    state);
spin_lock_irqsave(&q->lock, flags);
if (waitqueue_active(&q->midi_sleep))
/* The CPU might reorder the test for
   the waitqueue up here, before
   prior writes complete */
  if ((q->qlen>0 || q->head==q->tail)
  ...
  __ret = schedule_timeout(__ret)
if (q->qlen >= q->maxlen - 1) {
memcpy(&q->q[q->tail], ev, sizeof(*ev));
q->tail = (q->tail + 1) % q->maxlen;
q->qlen++;
------------------------------------------------------------------------

There are two other place in sound/core/seq/oss/ which have similar
code.  The attached patch removes the call to waitqueue_active() leaving
just wake_up() behind.  This fixes the problem because the call to
spin_lock_irqsave() in wake_up() will be an ACQUIRE operation.

I found this issue when I was looking through the linux source code
for places calling waitqueue_active() before wake_up*(), but without
preceding memory barriers, after sending a patch to fix a similar
issue in drivers/tty/n_tty.c  (Details about the original issue can be
found here: https://lkml.org/lkml/2015/9/28/849).

Signed-off-by: Kosuke Tatsukawa <tatsu@ab.jp.nec.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
sound/core/seq/oss/seq_oss_readq.c
sound/core/seq/oss/seq_oss_writeq.c