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Subject[016/102] rtc: fix hrtimer deadlock
3.0-stable review patch.  If anyone has any objections, please let us know.

------------------
From: Thomas Gleixner <tglx@linutronix.de>

commit b830ac1d9a2262093bb0f3f6a2fd2a1c8278daf5 upstream.

Ben reported a lockup related to rtc. The lockup happens due to:

CPU0 CPU1

rtc_irq_set_state() __run_hrtimer()
spin_lock_irqsave(&rtc->irq_task_lock) rtc_handle_legacy_irq();
spin_lock(&rtc->irq_task_lock);
hrtimer_cancel()
while (callback_running);
So the running callback never finishes as it's blocked on
rtc->irq_task_lock.

Use hrtimer_try_to_cancel() instead and drop rtc->irq_task_lock while
waiting for the callback. Fix this for both rtc_irq_set_state() and
rtc_irq_set_freq().

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reported-by: Ben Greear <greearb@candelatech.com>
Cc: John Stultz <john.stultz@linaro.org>
Cc: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

---
drivers/rtc/interface.c | 56 +++++++++++++++++++++++++++++++-----------------
1 file changed, 37 insertions(+), 19 deletions(-)
--- a/drivers/rtc/interface.c
+++ b/drivers/rtc/interface.c
@@ -636,6 +636,29 @@ void rtc_irq_unregister(struct rtc_devic
}
EXPORT_SYMBOL_GPL(rtc_irq_unregister);

+static int rtc_update_hrtimer(struct rtc_device *rtc, int enabled)
+{
+ /*
+ * We unconditionally cancel the timer here, because otherwise
+ * we could run into BUG_ON(timer->state != HRTIMER_STATE_CALLBACK);
+ * when we manage to start the timer before the callback
+ * returns HRTIMER_RESTART.
+ *
+ * We cannot use hrtimer_cancel() here as a running callback
+ * could be blocked on rtc->irq_task_lock and hrtimer_cancel()
+ * would spin forever.
+ */
+ if (hrtimer_try_to_cancel(&rtc->pie_timer) < 0)
+ return -1;
+
+ if (enabled) {
+ ktime_t period = ktime_set(0, NSEC_PER_SEC / rtc->irq_freq);
+
+ hrtimer_start(&rtc->pie_timer, period, HRTIMER_MODE_REL);
+ }
+ return 0;
+}
+
/**
* rtc_irq_set_state - enable/disable 2^N Hz periodic IRQs
* @rtc: the rtc device
@@ -651,24 +674,21 @@ int rtc_irq_set_state(struct rtc_device
int err = 0;
unsigned long flags;

+retry:
spin_lock_irqsave(&rtc->irq_task_lock, flags);
if (rtc->irq_task != NULL && task == NULL)
err = -EBUSY;
if (rtc->irq_task != task)
err = -EACCES;
- if (err)
- goto out;
-
- if (enabled) {
- ktime_t period = ktime_set(0, NSEC_PER_SEC/rtc->irq_freq);
- hrtimer_start(&rtc->pie_timer, period, HRTIMER_MODE_REL);
- } else {
- hrtimer_cancel(&rtc->pie_timer);
+ if (!err) {
+ if (rtc_update_hrtimer(rtc, enabled) < 0) {
+ spin_unlock_irqrestore(&rtc->irq_task_lock, flags);
+ cpu_relax();
+ goto retry;
+ }
+ rtc->pie_enabled = enabled;
}
- rtc->pie_enabled = enabled;
-out:
spin_unlock_irqrestore(&rtc->irq_task_lock, flags);
-
return err;
}
EXPORT_SYMBOL_GPL(rtc_irq_set_state);
@@ -690,20 +710,18 @@ int rtc_irq_set_freq(struct rtc_device *

if (freq <= 0)
return -EINVAL;
-
+retry:
spin_lock_irqsave(&rtc->irq_task_lock, flags);
if (rtc->irq_task != NULL && task == NULL)
err = -EBUSY;
if (rtc->irq_task != task)
err = -EACCES;
- if (err == 0) {
+ if (!err) {
rtc->irq_freq = freq;
- if (rtc->pie_enabled) {
- ktime_t period;
- hrtimer_cancel(&rtc->pie_timer);
- period = ktime_set(0, NSEC_PER_SEC/rtc->irq_freq);
- hrtimer_start(&rtc->pie_timer, period,
- HRTIMER_MODE_REL);
+ if (rtc->pie_enabled && rtc_update_hrtimer(rtc, 1) < 0) {
+ spin_unlock_irqrestore(&rtc->irq_task_lock, flags);
+ cpu_relax();
+ goto retry;
}
}
spin_unlock_irqrestore(&rtc->irq_task_lock, flags);



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