ALSA: pcm - fix race condition in wait_for_avail() (763437a9) · Commits · 戴 / test

wait_for_avail() in pcm_lib.c has a race in it (observed in practice by an Intel validation group). The function is supposed to return once space in the buffer has become available, or if some timeout happens. The entity that creates space (irq handler of sound driver and some such) will do a wake up on a waitqueue that this function registers for. However there are two races in the existing code 1) If space became available between the caller noticing there was no space and this function actually sleeping, the wakeup is missed and the timeout condition will happen instead 2) If a wakeup happened but not sufficient space became available, the code will loop again and wait for more space. However, if the second wake comes in prior to hitting the schedule_timeout_interruptible(), it will be missed, and potentially you'll wait out until the timeout happens. The fix consists of using more careful setting of the current state (so that if a wakeup happens in the main loop window, the schedule_timeout() falls through) and by checking for available space prior to going into the schedule_timeout() loop, but after being on the waitqueue and having the state set to interruptible. [tiwai: the following changes have been added to Arjan's original patch: - merged akpm's fix for waitqueue adding order into a single patch - reduction of duplicated code of avail check ] Signed-off-by:

Arjan van de Ven <arjan@linux.intel.com> Signed-off-by:

Andrew Morton <akpm@linux-foundation.org> Cc: <stable@kernel.org> Signed-off-by:

Takashi Iwai <tiwai@suse.de>

sound/core/pcm_lib.c

+24 −9

Original line number	Diff line number	Diff line
		@@ -1761,6 +1761,10 @@ static int wait_for_avail(struct snd_pcm_substream *substream,
		snd_pcm_uframes_t avail = 0;
		long wait_time, tout;

		init_waitqueue_entry(&wait, current);
		set_current_state(TASK_INTERRUPTIBLE);
		add_wait_queue(&runtime->tsleep, &wait);

		if (runtime->no_period_wakeup)
		wait_time = MAX_SCHEDULE_TIMEOUT;
		else {
		@@ -1771,16 +1775,32 @@ static int wait_for_avail(struct snd_pcm_substream *substream,
		}
		wait_time = msecs_to_jiffies(wait_time * 1000);
		}
		init_waitqueue_entry(&wait, current);
		add_wait_queue(&runtime->tsleep, &wait);

		for (;;) {
		if (signal_pending(current)) {
		err = -ERESTARTSYS;
		break;
		}

		/*
		* We need to check if space became available already
		* (and thus the wakeup happened already) first to close
		* the race of space already having become available.
		* This check must happen after been added to the waitqueue
		* and having current state be INTERRUPTIBLE.
		*/
		if (is_playback)
		avail = snd_pcm_playback_avail(runtime);
		else
		avail = snd_pcm_capture_avail(runtime);
		if (avail >= runtime->twake)
		break;
		snd_pcm_stream_unlock_irq(substream);
		tout = schedule_timeout_interruptible(wait_time);

		tout = schedule_timeout(wait_time);

		snd_pcm_stream_lock_irq(substream);
		set_current_state(TASK_INTERRUPTIBLE);
		switch (runtime->status->state) {
		case SNDRV_PCM_STATE_SUSPENDED:
		err = -ESTRPIPE;
		@@ -1806,14 +1826,9 @@ static int wait_for_avail(struct snd_pcm_substream *substream,
		err = -EIO;
		break;
		}
		if (is_playback)
		avail = snd_pcm_playback_avail(runtime);
		else
		avail = snd_pcm_capture_avail(runtime);
		if (avail >= runtime->twake)
		break;
		}
		_endloop:
		set_current_state(TASK_RUNNING);
		remove_wait_queue(&runtime->tsleep, &wait);
		*availp = avail;
		return err;

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