linux-kernel - Re: [RFC PATCHC 2/3] idle: store the idle state the cpu is

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Message-ID: <534D448A.8040002@linaro.org>
Date:	Tue, 15 Apr 2014 16:39:06 +0200
From:	Daniel Lezcano <daniel.lezcano@...aro.org>
To:	Peter Zijlstra <peterz@...radead.org>
CC:	linux-kernel@...r.kernel.org, mingo@...e.hu, rjw@...ysocki.net,
	nicolas.pitre@...aro.org, linux-pm@...r.kernel.org,
	alex.shi@...aro.org, vincent.guittot@...aro.org,
	morten.rasmussen@....com
Subject: Re: [RFC PATCHC 2/3] idle: store the idle state the cpu is

On 04/15/2014 04:33 PM, Peter Zijlstra wrote:
> On Tue, Apr 15, 2014 at 04:17:36PM +0200, Daniel Lezcano wrote:
>> On 04/15/2014 02:44 PM, Peter Zijlstra wrote:
>>> On Tue, Apr 15, 2014 at 02:43:30PM +0200, Peter Zijlstra wrote:
>>>> On Fri, Mar 28, 2014 at 01:29:55PM +0100, Daniel Lezcano wrote:
>>>>> @@ -143,6 +145,10 @@ static int cpuidle_idle_call(void)
>>>>>   			if (!ret) {
>>>>>   				trace_cpu_idle_rcuidle(next_state, dev->cpu);
>>>>>
>>>>> +				*power = &drv->states[next_state].power;
>>>>> +
>>>>> +				wmb();
>>>>> +
>>>>
>>>> I very much suspect you meant: smp_wmb(), as I don't see the hardware
>>>> reading that pointer, therefore UP wouldn't care. Also, any and all
>>>> barriers should come with a comment that describes the data ordering and
>>>> points to the matchin barriers.
>>>
>>> Furthermore, this patch fails to describe the life-time rules of the
>>> object placed there. Can the objected pointed to ever disappear?
>>
>> Hi Peter,
>>
>> thanks for reviewing the patches.
>>
>> There are a couple of situations where a cpuidle state can disappear:
>>
>> 1. For x86/acpi with dynamic c-states, when a laptop switches from battery
>> to AC that could result on removing the deeper idle state. The acpi driver
>> triggers:
>>
>> 'acpi_processor_cst_has_changed' which will call 'cpuidle_pause_and_lock'.
>> This one will call 'cpuidle_uninstall_idle_handler' which in turn calls
>> 'kick_all_cpus_sync'.
>>
>> All cpus will exit their idle state and the pointed object will be set to
>> NULL again.
>>
>> 2. The cpuidle driver is unloaded. Logically that could happen but not in
>> practice because the drivers are always compiled in and 95% of the drivers
>> are not coded to unregister the driver. Anyway ...
>>
>> The unloading code must call 'cpuidle_unregister_device', that calls
>> 'cpuidle_pause_and_lock' leading to 'kick_all_cpus_sync'.
>>
>> IIUC, the race can happen if we take the pointer and then one of these two
>> situation occurs at the same moment.
>>
>> As the function 'find_idlest_cpu' is inside a rcu_read_lock may be a
>> rcu_barrier in 'cpuidle_pause_and_lock' or 'cpuidle_uninstall_idle_handler'
>> should suffice, no ?
>
> Indeed. But be sure to document this.

Yes, sure. Thanks for pointing this.

   -- Daniel


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