From 694470273de29c2d3f3792856d4e748969294789 Mon Sep 17 00:00:00 2001 From: Kosuke Tatsukawa Date: Fri, 9 Oct 2015 00:35:54 +0000 Subject: [PATCH] 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 Signed-off-by: Takashi Iwai --- 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 ccd893566f1d..046cb586fb2f 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 d50338bbc21f..1f6788a18444 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); }