linux-kernel - Re: [PATCH 1/2] timer: Avoid waking up an idle-core by migrate running timer

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Date:	Wed, 15 Apr 2015 17:54:34 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Viresh Kumar <viresh.kumar@...aro.org>
cc:	Ingo Molnar <mingo@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	linaro-kernel@...ts.linaro.org, linux-kernel@...r.kernel.org,
	Steven Miao <realmz6@...il.com>
Subject: Re: [PATCH 1/2] timer: Avoid waking up an idle-core by migrate
 running timer

On Tue, 31 Mar 2015, Viresh Kumar wrote:
> @@ -1213,6 +1249,25 @@ static inline void __run_timers(struct tvec_base *base)
>  				call_timer_fn(timer, fn, data);
>  				spin_lock_irq(&base->lock);
>  			}
> +
> +			/*
> +			 * Handler running on this base, re-queued itself on
> +			 * another base ?
> +			 */
> +			if (unlikely(timer->base != base)) {
> +				unsigned long flags;
> +				struct tvec_base *tbase;
> +
> +				spin_unlock(&base->lock);
> +
> +				tbase = lock_timer_base(timer, &flags);
> +				timer_clear_running(timer);
> +				spin_unlock(&tbase->lock);
> +
> +				spin_lock(&base->lock);
> +			} else {
> +				timer_clear_running(timer);
> +			}

And just for the record:

Dereferencing timer _AFTER_ the callback function is a big NONO. The
callback function is allowed to free the timer. See the comment in
call_timer_fn()

Oh well,

	tglx
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