Message-ID: <CAKohponYvbpaphKCHoPH=6jDCpWop_BH_DKq6HwOx0SEe39sqw@mail.gmail.com>
Date:	Thu, 16 Oct 2014 16:53:52 +0530
From:	Viresh Kumar <viresh.kumar@...aro.org>
To:	Prarit Bhargava <prarit@...hat.com>
Cc:	Saravana Kannan <skannan@...eaurora.org>,
	"Rafael J. Wysocki" <rjw@...ysocki.net>,
	"linux-pm@...r.kernel.org" <linux-pm@...r.kernel.org>,
	Linux Kernel <linux-kernel@...r.kernel.org>,
	Robert Schöne <robert.schoene@...dresden.de>
Subject: Re: Locking issues with cpufreq and sysfs

On 14 October 2014 23:54, Prarit Bhargava <prarit@...hat.com> wrote:
> Here's what I think we should do.  Taking a step back, the purpose of the
> cpufreq sysfs files is to allow userspace to read current cpu frequency
> settings, and to allow userspce to modify the governor and set the max & min
> ranges for cpu frequencies.  This can be done per device or for all cpus
> depending on the driver.

Okay.

> We have to guarantee that bothing reading and writing will always work and that
> write operations will always be atomic relative to userspace.  The current

Ok.

> implementation of cpufreq does this through the following locks:
>
> cpufreq_driver_lock: protects the cpufreq_cpu_data array and cpufreq_driver->boost
> cpufreq_governor_lock: protects the current governor

Its just for serialization..

> cpufreq_governor_mutex: protects the cpufreq_governor_list
> cpufreq_rwsem: protects the driver from being unloaded
> global_kobj_lock: protects the "cpufreq" kobject
> each policy has a rwsem (policy->rwsem): protects the cpufreq_policy struct
> each policy has a transition_lock (policy->transition): synchronizes
>                                                         frequency transitions
>
> While examining this code I was wondering exactly why we allow multiple readers
> and writers in cpufreq.  I could understand if we felt that this data was
> critical; but it really isn't.  A short delay here isn't that big of a deal IMO
> (if someone can produce a case where a delay would cause a serious problem I'd
>  like to hear it).  I don't even think it is safe in most cases to allow readers
> while cpufreq values are changing; if we're changing the governor userspace
> cannot rely on the value of (for example) cpuinfo_max_freq.

I don't know how reader writer lock will fail and a normal lock will not.
There is only benefit of rwlock, that readers can read things while
there is nobody
writing..

> So I'm proposing that we move to a single threaded read/write using, if

Okay, but how will that benefit us ?

> possible, a single policy lock for now.  We might transition this back to a
> rwsem later on, however, for the first attempt at cleaning this up I think we
> should just stick with a simple lock.  In doing that, IMO we remove
>
> cpufreq_rwsem: protects the driver from being unloaded
> cpufreq_governor_lock: protects the current governor
> each policy has a rwsem (policy->rwsem): protects the cpufreq_policy struct
>
> and potentially
>
> cpufreq_driver_lock: protects the cpufreq_cpu_data array and cpufreq_driver->boost

Not really sure, but yeah we might be able to club few of them..

> After looking at the way the code would be structured, I'm wondering if
>
> cpufreq_governor_mutex: protects the cpufreq_governor_list
>
> is overkill.  The loading of a module should be atomic relative to the cpufreq
> code, so this lock may not be required.  (Admittedly I haven't tested that...)
>
> That would leave:
>
> global_kobj_lock: protects the "cpufreq" kobject
> each policy has a transition_lock (policy->transition): synchronizes
>                                                         frequency transitions
>
> and a new lock, perhaps called policy->lock, to serialize all events.
>
> Pros: We clean all this up to a simpler single threaded model.  Bugs and races
> here would be much easier to handle.  We're currently putting band-aid on
> band-aids in this code ATM and it looks like we're seeing old races expanded
> or new races exposed.
>
> Cons: We lose the ability to do simultaneous reads and writes ... although
> I remain unconvinced that this would ever be safe to do.  ie) If I change the
> governor while at the same time reading, for example, the current cpu
> frequency I cannot rely on that value to be valid.
>
> After that we can add some reference counting to the sysfs file accesses
> so that we can block after the sysfs removal when we change cpufreq
> governors.  I think that would be trivial and that it would resolve any races
> when adding and removing governor's sysfs files.

Not really sure, but if you solve few things with getting these bugs resolved
then we might apply your patches without any issues.
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