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SubjectRe: [patch 2/3] rtc: Fix hrtimer deadlock
On Fri, 22 Jul 2011 09:12:51 -0000
Thomas Gleixner <tglx@linutronix.de> wrote:
> 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().
>
> ...
>
> +static int rtc_update_hrtimer(struct rtc_device *rtc, int enabled)
> +{
> + /*
> + * We unconditionally cancel the timer here, because otherwise

The comment seems wrong. If hrtimer_try_to_cancel() fails, we simply
bale out so we did not "unconditionally cancel the timer"?

> + * 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;

Well this is rather nasty. Sort of an open-coded expensive spinlock.
All rather pointless on SMP=n builds, too.

Is there no better way, such as fixing up the locking properly?



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