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>
---
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
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