linux-kernel - Re: [PATCH] ARM: timer: Shutdown clock event device when stopping local timer

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Date:	Mon, 01 Apr 2013 02:06:17 +0200
From:	Daniel Lezcano <daniel.lezcano@...aro.org>
To:	ning.n.jiang@...il.com
CC:	linux@....linux.org.uk, kgene.kim@...sung.com,
	davidb@...eaurora.org, dwalker@...o99.com, bryanh@...eaurora.org,
	john.stultz@...aro.org, tglx@...utronix.de,
	linus.walleij@...aro.org, shawn.guo@...aro.org,
	rob.herring@...xeda.com, arnd@...db.de,
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
	linux-samsung-soc@...r.kernel.org, linux-arm-msm@...r.kernel.org
Subject: Re: [PATCH] ARM: timer: Shutdown clock event device when stopping
 local timer

On 03/29/2013 10:24 AM, ning.n.jiang@...il.com wrote:
> From: Ning Jiang <ning.n.jiang@...il.com>
> 
> Currently there are two problems when we try to stop local timer.
> First, it calls set_mode function directly so mode state is not
> updated for the clock event device. Second, it makes the device
> unused instead of shutdown.
> 
> A subtle error will happen because of it. When a cpu is plugged out
> it will stop the local timer. It will call tick_nohz_idle_enter()
> in idle thread afterwards. It will cancel the sched timer and try
> to reprogram the next event. This is wrong since the local timer
> is supposed to be stopped.
> 
> The right way to stop the local timer is to shutdown it by calling
> clockevents_set_mode(). Thus when we try to reprogram the clock
> event device, it will return directly without doing anything since
> the clock mode is CLOCK_EVT_MODE_SHUTDOWN.
> 
> Signed-off-by: Ning Jiang <ning.n.jiang@...il.com>
> ---


Don't you see a kernel BUG with this patch ?

In the clockevents code, there is:

/**
 * clockevents_notify - notification about relevant events
 */
void clockevents_notify(unsigned long reason, void *arg)
{
	...
	case CLOCK_EVT_NOTIFY_CPU_DEAD:
	...
                cpu = *((int *)arg);
                list_for_each_entry_safe(dev, tmp, &clockevent_devices,
					list) {
                       if (cpumask_test_cpu(cpu, dev->cpumask) &&
                            cpumask_weight(dev->cpumask) == 1 &&
                            !tick_is_broadcast_device(dev)) {
                                BUG_ON(dev->mode !=
					CLOCK_EVT_MODE_UNUSED);

				^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

                                list_del(&dev->list);
                        }
                }
                break;
	...
}

This is called triggered from hrtimer_cpu_notify with the CPU_DEAD event.


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