linux-kernel - Re: [PATCH 08/14] hrtimer: Allow hrtimer::function() to free the timer

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Date:	Mon, 8 Jun 2015 00:56:38 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	umgwanakikbuti@...il.com, mingo@...e.hu, ktkhai@...allels.com,
	rostedt@...dmis.org, tglx@...utronix.de, juri.lelli@...il.com,
	pang.xunlei@...aro.org, wanpeng.li@...ux.intel.com,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 08/14] hrtimer: Allow hrtimer::function() to free the
	timer

On 06/08, Oleg Nesterov wrote:
>
> And I simply can not understand the complication in hrtimer_active(),
> please help!

Sorry for another off-topic email, but I don't even understand the
usage of hrtimer_active().

Say, do_nanosleep()

		hrtimer_start_expires(&t->timer, mode);
		if (!hrtimer_active(&t->timer))
			t->task = NULL;

why? Assuming that hrtimer_active() is correct, it can only return
false if t->task was already cleared by hrtimer_wakeup().


OTOH. perf_cpu_hrtimer_restart() does

	if (hrtimer_active(hr))
		return;

	if (!hrtimer_callback_running(hr))
		__hrtimer_start_range_ns(...);

why it can't simply do

	if (!hrtimer_active(hr)) // implies !hrtimer_callback_running()
		__hrtimer_start_range_ns(...);


Confused.

Oleg.

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