rtc: Fix hrtimer deadlock

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().

Cc: stable@kernel.org
Reported-by: Ben Greear <greearb@candelatech.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: John Stultz <john.stultz@linaro.org>

1 files changed, 37 insertions, 19 deletions

diff --git a/drivers/rtc/interface.c b/drivers/rtc/interface.c
index b6bf57f25cc9..a1ba2caa8308 100644
--- a/drivers/rtc/interface.c
+++ b/drivers/rtc/interface.c
@@ -636,6 +636,29 @@ void rtc_irq_unregister(struct rtc_device *rtc, struct rtc_task *task)
 }
 EXPORT_SYMBOL_GPL(rtc_irq_unregister);
 
+static int rtc_update_hrtimer(struct rtc_device *rtc, int enabled)
+{
+        /*
+         * We always cancel the timer here first, 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 *rtc, struct rtc_task *task, int enabled
         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 *rtc, struct rtc_task *task, int freq)
 
         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);