Re: [RFC PATCH 11/19] cpufreq: assert policy->rwsem is held in __cpufreq_governor

[Saravana Kannan]

On 01/12/2016 02:20 AM, Viresh Kumar wrote:
> On 11-01-16, 17:35, Juri Lelli wrote:
> > __cpufreq_governor works on policy, so policy->rwsem has to be held.
> > Add assertion for such condition.
> >
> > Cc: "Rafael J. Wysocki" <rjw@xxxxxxxxxxxxx>
> > Cc: Viresh Kumar <viresh.kumar@xxxxxxxxxx>
> > Signed-off-by: Juri Lelli <juri.lelli@xxxxxxx>
> > ---
> >   drivers/cpufreq/cpufreq.c | 3 +++
> >   1 file changed, 3 insertions(+)
> >
> > diff --git a/drivers/cpufreq/cpufreq.c b/drivers/cpufreq/cpufreq.c
> > index f1f9fbc..e7fc5c9 100644
> > --- a/drivers/cpufreq/cpufreq.c
> > +++ b/drivers/cpufreq/cpufreq.c
> > @@ -1950,6 +1950,9 @@ static int __cpufreq_governor(struct cpufreq_policy *policy,
> >   	/* Don't start any governor operations if we are entering suspend */
> >   	if (cpufreq_suspended)
> >   		return 0;
> > +
> > +	lockdep_assert_held(&policy->rwsem);
> > +
>
> We had an ABBA problem with the EXIT governor callback and so this
> rwsem is dropped just before that from set_policy()..
>
> commit 955ef4833574 ("cpufreq: Drop rwsem lock around
> CPUFREQ_GOV_POLICY_EXIT")

AFAIR, the ABBA issue was between the sysfs lock and the policy lock. 
The fix for that issue should not be dropping the lock around 
POLICY_EXIT. The proper fix is to have the governor "export" the 
attributes it wants to add/remove and have the cpufreq framework do the 
adding/removing of the attributes from sysfs for the governor.

Thanks,
Saravana

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