> -----Original Message-----
> From: linux-kernel-ow...@vger.kernel.org [mailto:linux-kernel-
> ow...@vger.kernel.org] On Behalf Of Prarit Bhargava
> Sent: Tuesday, 14 October, 2014 1:24 PM
> To: Viresh Kumar
> Cc: Saravana Kannan; Rafael J. Wysocki; linux...@vger.kernel.org; Linux
> Kernel; Robert Schöne
> Subject: Re: Locking issues with cpufreq and sysfs
>
> On 10/14/2014 03:10 AM, Viresh Kumar wrote:
> > On 13 October 2014 18:41, Prarit Bhargava <pra...@redhat.com> wrote:
> >>
> >> The locking is insufficient here, Viresh. I no longer believe that fixes
> >> to this locking scheme are the right way to move forward here. I'm
> wondering
> >> if we can look at other alternatives such as maintaining a refcount or
> >> perhaps using a queuing mechanism for governor and policy related changes.
> >>
...
> So I'm proposing that we move to a single threaded read/write using, if
> 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
>
> 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.
>
Please keep performance in mind too. cpufreq_governor_lock
contention is a bit of an issue with heavy IO workloads
as described in:
http://marc.info/?l=linux-pm&m=140924051503827&w=2
---
Rob Elliott HP Server Storage
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