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Subject[PATCH 3.12 71/88] ALSA: hrtimer: Handle start/stop more properly
Date
From: Takashi Iwai <tiwai@suse.de>

3.12-stable review patch. If anyone has any objections, please let me know.

===============

commit d2c5cf88d5282de258f4eb6ab40040b80a075cd8 upstream.

This patch tries to address the still remaining issues in ALSA hrtimer
driver:
- Spurious use-after-free was detected in hrtimer callback
- Incorrect rescheduling due to delayed start
- WARN_ON() is triggered in hrtimer_forward() invoked in hrtimer
callback

The first issue happens only when the new timer is scheduled even
while hrtimer is being closed. It's related with the second and third
items; since ALSA timer core invokes hw.start callback during hrtimer
interrupt, this may result in the explicit call of hrtimer_start().

Also, the similar problem is seen for the stop; ALSA timer core
invokes hw.stop callback even in the hrtimer handler, too. Since we
must not call the synced hrtimer_cancel() in such a context, it's just
a hrtimer_try_to_cancel() call that doesn't properly work.

Another culprit of the second and third items is the call of
hrtimer_forward_now() before snd_timer_interrupt(). The timer->stick
value may change during snd_timer_interrupt() call, but this
possibility is ignored completely.

For covering these subtle and messy issues, the following changes have
been done in this patch:
- A new flag, in_callback, is introduced in the private data to
indicate that the hrtimer handler is being processed.
- Both start and stop callbacks skip when called from (during)
in_callback flag.
- The hrtimer handler returns properly HRTIMER_RESTART and NORESTART
depending on the running state now.
- The hrtimer handler reprograms the expiry properly after
snd_timer_interrupt() call, instead of before.
- The close callback clears running flag and sets in_callback flag
to block any further start/stop calls.

Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Jiri Slaby <jslaby@suse.cz>
---
sound/core/hrtimer.c | 56 ++++++++++++++++++++++++++++++++++++----------------
1 file changed, 39 insertions(+), 17 deletions(-)

diff --git a/sound/core/hrtimer.c b/sound/core/hrtimer.c
index 14d483d6b3b0..327bbf797c6c 100644
--- a/sound/core/hrtimer.c
+++ b/sound/core/hrtimer.c
@@ -38,37 +38,53 @@ static unsigned int resolution;
struct snd_hrtimer {
struct snd_timer *timer;
struct hrtimer hrt;
- atomic_t running;
+ bool in_callback;
};

static enum hrtimer_restart snd_hrtimer_callback(struct hrtimer *hrt)
{
struct snd_hrtimer *stime = container_of(hrt, struct snd_hrtimer, hrt);
struct snd_timer *t = stime->timer;
- unsigned long oruns;
-
- if (!atomic_read(&stime->running))
- return HRTIMER_NORESTART;
-
- oruns = hrtimer_forward_now(hrt, ns_to_ktime(t->sticks * resolution));
- snd_timer_interrupt(stime->timer, t->sticks * oruns);
+ ktime_t delta;
+ unsigned long ticks;
+ enum hrtimer_restart ret = HRTIMER_NORESTART;
+
+ spin_lock(&t->lock);
+ if (!t->running)
+ goto out; /* fast path */
+ stime->in_callback = true;
+ ticks = t->sticks;
+ spin_unlock(&t->lock);
+
+ /* calculate the drift */
+ delta = ktime_sub(hrt->base->get_time(), hrtimer_get_expires(hrt));
+ if (delta.tv64 > 0)
+ ticks += ktime_divns(delta, ticks * resolution);
+
+ snd_timer_interrupt(stime->timer, ticks);
+
+ spin_lock(&t->lock);
+ if (t->running) {
+ hrtimer_add_expires_ns(hrt, t->sticks * resolution);
+ ret = HRTIMER_RESTART;
+ }

- if (!atomic_read(&stime->running))
- return HRTIMER_NORESTART;
- return HRTIMER_RESTART;
+ stime->in_callback = false;
+ out:
+ spin_unlock(&t->lock);
+ return ret;
}

static int snd_hrtimer_open(struct snd_timer *t)
{
struct snd_hrtimer *stime;

- stime = kmalloc(sizeof(*stime), GFP_KERNEL);
+ stime = kzalloc(sizeof(*stime), GFP_KERNEL);
if (!stime)
return -ENOMEM;
hrtimer_init(&stime->hrt, CLOCK_MONOTONIC, HRTIMER_MODE_REL);
stime->timer = t;
stime->hrt.function = snd_hrtimer_callback;
- atomic_set(&stime->running, 0);
t->private_data = stime;
return 0;
}
@@ -78,6 +94,11 @@ static int snd_hrtimer_close(struct snd_timer *t)
struct snd_hrtimer *stime = t->private_data;

if (stime) {
+ spin_lock_irq(&t->lock);
+ t->running = 0; /* just to be sure */
+ stime->in_callback = 1; /* skip start/stop */
+ spin_unlock_irq(&t->lock);
+
hrtimer_cancel(&stime->hrt);
kfree(stime);
t->private_data = NULL;
@@ -89,18 +110,19 @@ static int snd_hrtimer_start(struct snd_timer *t)
{
struct snd_hrtimer *stime = t->private_data;

- atomic_set(&stime->running, 0);
- hrtimer_try_to_cancel(&stime->hrt);
+ if (stime->in_callback)
+ return 0;
hrtimer_start(&stime->hrt, ns_to_ktime(t->sticks * resolution),
HRTIMER_MODE_REL);
- atomic_set(&stime->running, 1);
return 0;
}

static int snd_hrtimer_stop(struct snd_timer *t)
{
struct snd_hrtimer *stime = t->private_data;
- atomic_set(&stime->running, 0);
+
+ if (stime->in_callback)
+ return 0;
hrtimer_try_to_cancel(&stime->hrt);
return 0;
}
--
2.9.1
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