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Date:	Fri, 11 Dec 2009 10:29:27 +0800
From:	Xiaotian Feng <dfeng@...hat.com>
To:	Thomas Gleixner <tglx@...utronix.de>
CC:	damm@...l.co.jp, hsweeten@...ionengravers.com,
	akpm@...ux-foundation.org, venkatesh.pallipadi@...el.com,
	linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH 0/4] clockevents: fix clockevent_devices list corruption
 after cpu hotplug

On 12/10/2009 10:35 PM, Thomas Gleixner wrote:
> On Thu, 10 Dec 2009, Xiaotian Feng wrote:
>> I've met a list_del corruption, which was reported in
>> http://lkml.org/lkml/2009/11/27/45. But no response, so I try to debug it
>> by myself.
>>
>> After I added some printks to show all elements in clockevent_devices, I
>> found kernel hangs when I tried to resume from s2ram.
>>
>> In clockevents_register_device, clockevents_do_notify ADD is always followed
>> by clockevents_notify_released. Although clockevents_do_notify ADD will use
>> tick_check_new_device to add new devices and replace old devices to the
>> clockevents_released list, clockevents_notify_released add them back to
>> clockevent_devices list.
>>
>> My system is Quad-Core x86_64, with apic and hpet enables, after boot up,
>> the elements in clockevent_devices list is :
>> clockevent_device->lapic(3)->hpet5(3)->lapic(2)->hpet4(2)->lapic(1)->hpet3(1)-
>>    ->lapic(0)->hpet2(0)->hpet(0)
>> * () means cpu id
>>
>> But active clock_event_device is hpet2,hpet3,hpet4,hpet5. Then at s2ram stage,
>> cpu 1,2,3 is down, then notify CLOCK_EVT_NOTIFY_CPU_DEAD will calls tick_shutdown,
>> then hpet2,hpet3,hpet4,hpet5 was deleted from clockevent_device list.
>> So after s2ram, elements in clockevent_device list is:
>> clockevent_device->lapic(3)->lapic(2)->lapic(1)->lapic(0)->hpet2(0)->hpet(0)
>>
>> Then at resume stage, cpu 1,2,3 is up, it will register lapic again, and then
>> perform list_add lapic on clockevent_device list, e.g. list_add lapic(1) on
>> above list, lapic will move to the clockevent_device->next, but lapic(2)->next
>> is still point to lapic(1), the list is circular and corrupted then.
>
> Great detective work !
>
>> This patchset aims to fixes above behaviour by:
>>         - on clockevents_register_device, if notify ADD success, move new devices
>>           to the clockevent_devices list, otherwise move to clockevents_released
>>           list.
>>         - on clockevents_notify_released, same behaviour as above.
>>         - on clockevents_notify CPU_DEAD, remove related devices on dead cpu from
>>           clockevents_released list.
>>
>> It makes sure that only active devices on each cpu is on clockevent_devices list.
>> With this patchset, the list_del corruption disappeared, and suspend/resume, cpu
>> hotplug works fine on my system.
>
> I'm not happy about that churn. Why don't we simply scan the
> clockevent_devices list for leftovers of the dead CPU ?
My only thought is to make clockevent_devices list only store active 
devices on each cpu, all other inactive devices stored on 
clockevents_released, but this make things complex. Your patch is better.
>
> Untested patch below solves the same problem.
Yes, this also resolves my list_del warning. Thanks
>
> Thanks,
>
> 	tglx
> ----
> diff --git a/kernel/time/clockevents.c b/kernel/time/clockevents.c
> index 20a8920..5dd857f 100644
> --- a/kernel/time/clockevents.c
> +++ b/kernel/time/clockevents.c
> @@ -238,8 +238,9 @@ void clockevents_exchange_device(struct clock_event_device *old,
>    */
>   void clockevents_notify(unsigned long reason, void *arg)
>   {
> -	struct list_head *node, *tmp;
> +	struct clock_event_device *dev, *tmp;
>   	unsigned long flags;
> +	int cpu;
>
>   	spin_lock_irqsave(&clockevents_lock, flags);
>   	clockevents_do_notify(reason, arg);
> @@ -250,8 +251,19 @@ void clockevents_notify(unsigned long reason, void *arg)
>   		 * Unregister the clock event devices which were
>   		 * released from the users in the notify chain.
>   		 */
> -		list_for_each_safe(node, tmp,&clockevents_released)
> -			list_del(node);
> +		list_for_each_entry_safe(dev, tmp,&clockevents_released, list)
> +			list_del(&dev->list);
> +		/*
> +		 * Now check whether the CPU has left unused per cpu devices
> +		 */
> +		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) {
> +				BUG_ON(dev->mode != CLOCK_EVT_MODE_UNUSED);
> +				list_del(&dev->list);
> +			}
> +		}
>   		break;
>   	default:
>   		break;
>
>
>

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