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Date:	Wed, 24 Dec 2014 09:44:45 -0800
From:	Dmitry Torokhov <dtor@...omium.org>
To:	Nishanth Menon <nm@...com>
Cc:	Dmitry Torokhov <dtor@...omium.org>,
	"Rafael J. Wysocki" <rjw@...ysocki.net>,
	Viresh Kumar <viresh.kumar@...aro.org>,
	Thomas Petazzoni <thomas.petazzoni@...e-electrons.com>,
	Geert Uytterhoeven <geert+renesas@...der.be>,
	Stefan Wahren <stefan.wahren@...e.com>,
	Paul Gortmaker <paul.gortmaker@...driver.com>,
	"linux-pm@...r.kernel.org" <linux-pm@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 3/4] PM / OPP: take RCU lock in dev_pm_opp_get_opp_count

On Wed, Dec 24, 2014 at 9:37 AM, Nishanth Menon <nm@...com> wrote:
> On 12/24/2014 11:31 AM, Dmitry Torokhov wrote:
>> On Wed, Dec 24, 2014 at 9:16 AM, Nishanth Menon <nm@...com> wrote:
>>> On 12/24/2014 11:09 AM, Dmitry Torokhov wrote:
>>>> On Wed, Dec 24, 2014 at 8:48 AM, Nishanth Menon <nm@...com> wrote:
>>>>> On 12/16/2014 05:09 PM, Dmitry Torokhov wrote:
>>>>>> A lot of callers are missing the fact that dev_pm_opp_get_opp_count
>>>>>> needs to be called under RCU lock. Given that RCU locks can safely be
>>>>>> nested, instead of providing *_locked() API, let's take RCU lock inside
>>>>>> dev_pm_opp_get_opp_count() and leave callers as is.
>>>>>
>>>>> While it is true that we can safely do nested RCU locks, This also
>>>>> encourages wrong usage.
>>>>>
>>>>> count = dev_pm_opp_get_opp_count(dev)
>>>>> ^^ point A
>>>>> array   = kzalloc(count * sizeof (*array));
>>>>> rcu_read_lock();
>>>>> ^^ point B
>>>>> .. work down the list and add OPPs..
>>>>> ...
>>>>>
>>>>> Between A and B, we might have had list modification (dynamic OPP
>>>>> addition or deletion) - which implies that the count is no longer
>>>>> accurate between point A and B. instead, enforcing callers to have the
>>>>> responsibility of rcu_lock is exactly what we have to do since the OPP
>>>>> library has no clue how to enforce pointer or data accuracy.
>>>>
>>>> No, you seem to have a misconception that rcu_lock protects you past
>>>> the point B, but that is also wrong. The only thing rcu "lock"
>>>> provides is safe traversing the list and guarantee that elements will
>>>> not disappear while you are referencing them, but list can both
>>>> contract and expand under you. In that regard code in
>>>> drivers/cpufreq/cpufreq_opp.c is utterly wrong. If you want to count
>>>> the list and use number of elements you should be taking a mutex.
>>>> Luckily all cpufreq drivers at the moment only want to see if OPP
>>>> table is empty or not, so as a stop-gap we can take rcu_lock
>>>> automatically as we are getting count. We won't get necessarily
>>>> accurate result, but at least we will be safe traversing the list.
>>>
>>> So, instead of a half solution, lets consider this in the realm of
>>> dynamic OPPs as well. agreed to the point that we only have safe
>>> traversal and pointer validity. the real problem however is with
>>> "dynamic OPPs" (one of the original reasons why i did not add dynamic
>>> OPPs in the original version was to escape from it's complexity for
>>> users - anyways.. we are beyond that now). if OPPs can be removed on
>>> the fly, we need the following:
>>> a) use OPP notifiers to adequately handle list modification
>>> b) lock down list modification (and associated APIs) to ensure that
>>> the original cpufreq /devfreq list is correct.
>>>
>>> I still dont see the need to do this half solution.
>>
>> The need for half solution at the moment is that you can't safely
>> travel the lists and may crash on an invalid pointer.
>
> So, fix the cpufreq-dt instead of moving the hack inside OPP driver.

I started there, but it is not only cpufreq-dt that got it wrong. I
considered changing individual drivers (Viresh also suggested adding
_locked() variant API), but decided patching opp was less invasive for
now.

>
>>
>> Going forward I think (I mentioned that in my other email) that we
>> should rework the OPP API so that callers fetch OPP table object for a
>> device at init/probe time and then use it to get OPPs. This way won't
>> have to travel two lists any time we want to reference an OPP.
>>
>> And instead of relying notifiers, maybe look into using OPP tables
>> directly in cpufreq drivers instead of converting OPP into static-ish
>> cpufreq tables.
>>
>
> If you'd like a proper fix for OPP usage, I am all open to see such a
> proposal that works not just for cpufreq, but also for devfreq as well.

Yeah, let's see what kind of time I have ;)

Thanks.

-- 
Dmitry
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