On Fri, 09 Oct 2015 02:35:54 +0200, Kosuke Tatsukawa wrote:

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

Thanks, applied now.

Takashi

---

sound/core/seq/oss/seq_oss_readq.c | 6 ++---- sound/core/seq/oss/seq_oss_writeq.c | 4 +--- 2 files changed, 3 insertions(+), 7 deletions(-)

diff --git a/sound/core/seq/oss/seq_oss_readq.c b/sound/core/seq/oss/seq_oss_readq.c index ccd8935..046cb58 100644 --- a/sound/core/seq/oss/seq_oss_readq.c +++ b/sound/core/seq/oss/seq_oss_readq.c @@ -91,8 +91,7 @@ snd_seq_oss_readq_clear(struct seq_oss_readq *q) q->head = q->tail = 0; } /* if someone sleeping, wake'em up */

• if (waitqueue_active(&q->midi_sleep))

• wake_up(&q->midi_sleep);
  

• wake_up(&q->midi_sleep); q->input_time = (unsigned long)-1;

}

@@ -138,8 +137,7 @@ snd_seq_oss_readq_put_event(struct seq_oss_readq *q, union evrec *ev) q->qlen++;

/* wake up sleeper */

• if (waitqueue_active(&q->midi_sleep))

• wake_up(&q->midi_sleep);
  

• wake_up(&q->midi_sleep);

  spin_unlock_irqrestore(&q->lock, flags);

diff --git a/sound/core/seq/oss/seq_oss_writeq.c b/sound/core/seq/oss/seq_oss_writeq.c index d50338b..1f6788a 100644 --- a/sound/core/seq/oss/seq_oss_writeq.c +++ b/sound/core/seq/oss/seq_oss_writeq.c @@ -138,9 +138,7 @@ snd_seq_oss_writeq_wakeup(struct seq_oss_writeq *q, abstime_t time) spin_lock_irqsave(&q->sync_lock, flags); q->sync_time = time; q->sync_event_put = 0;

• if (waitqueue_active(&q->sync_sleep)) {

• wake_up(&q->sync_sleep);
  

• }

• wake_up(&q->sync_sleep); spin_unlock_irqrestore(&q->sync_lock, flags);

}

-- 1.7.1