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>
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@ -91,8 +91,7 @@ snd_seq_oss_readq_clear(struct seq_oss_readq *q)
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q->head = q->tail = 0;
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}
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/* if someone sleeping, wake'em up */
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if (waitqueue_active(&q->midi_sleep))
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wake_up(&q->midi_sleep);
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wake_up(&q->midi_sleep);
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q->input_time = (unsigned long)-1;
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}
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@ -138,8 +137,7 @@ snd_seq_oss_readq_put_event(struct seq_oss_readq *q, union evrec *ev)
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q->qlen++;
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/* wake up sleeper */
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if (waitqueue_active(&q->midi_sleep))
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wake_up(&q->midi_sleep);
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wake_up(&q->midi_sleep);
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spin_unlock_irqrestore(&q->lock, flags);
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@ -138,9 +138,7 @@ snd_seq_oss_writeq_wakeup(struct seq_oss_writeq *q, abstime_t time)
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spin_lock_irqsave(&q->sync_lock, flags);
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q->sync_time = time;
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q->sync_event_put = 0;
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if (waitqueue_active(&q->sync_sleep)) {
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wake_up(&q->sync_sleep);
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}
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wake_up(&q->sync_sleep);
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spin_unlock_irqrestore(&q->sync_lock, flags);
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}
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