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 <ta...@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
>
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majord...@vger.kernel.org
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/
