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SubjectRe: [RFC PATCH 05/19] cpufreq: assert locking when accessing cpufreq_policy_list
Hi,

On 12/01/16 15:04, Viresh Kumar wrote:
> On 11-01-16, 17:35, Juri Lelli wrote:
> > cpufreq_policy_list is guarded by cpufreq_driver_lock. Add appropriate
> > locking assertions to check that we always access the list while holding
> > the associated lock.
> >
> > Cc: "Rafael J. Wysocki" <rjw@rjwysocki.net>
> > Cc: Viresh Kumar <viresh.kumar@linaro.org>
> > Signed-off-by: Juri Lelli <juri.lelli@arm.com>
> > ---
> > drivers/cpufreq/cpufreq.c | 3 +++
> > 1 file changed, 3 insertions(+)
> >
> > diff --git a/drivers/cpufreq/cpufreq.c b/drivers/cpufreq/cpufreq.c
> > index 00a00cd..63d6efb 100644
> > --- a/drivers/cpufreq/cpufreq.c
> > +++ b/drivers/cpufreq/cpufreq.c
> > @@ -65,6 +65,7 @@ static bool suitable_policy(struct cpufreq_policy *policy, bool active)
> > static struct cpufreq_policy *next_policy(struct cpufreq_policy *policy,
> > bool active)
> > {
> > + lockdep_assert_held(&cpufreq_driver_lock);
> > do {
> > policy = list_next_entry(policy, policy_list);
> >
> > @@ -80,6 +81,7 @@ static struct cpufreq_policy *first_policy(bool active)
> > {
> > struct cpufreq_policy *policy;
> >
> > + lockdep_assert_held(&cpufreq_driver_lock);
>
> Because both first_policy() and next_policy() are parts of
> for_each_suitable_policy() macro, checking this in first_policy() is
> sufficient. next_policy() isn't designed to be used by any other code.
>

But next_policy is called multiple times as part of
for_each_suitable_policy(). What if someone thinks she/he can release
cpufreq_driver_lock inside for_each_(in)active_policy() loop? Not that
it makes sense, but don't you think it could happen?

> > /* No policies in the list */
> > if (list_empty(&cpufreq_policy_list))
> > return NULL;
> > @@ -2430,6 +2432,7 @@ int cpufreq_register_driver(struct cpufreq_driver *driver_data)
> > if (ret)
> > goto err_boost_unreg;
> >
> > + lockdep_assert_held(&cpufreq_driver_lock);
>
> Why do you need a cpufreq_driver_lock here? And the above change
> should generate a lockdep here as the lock isn't taken right now.
>

Because you are checking cpufreq_policy_list to see if it's empty. And
it generates a lockdep warning, yes; fixed by next patch. Maybe putting
fixes before warnings, as you are suggesting, is better.

Thanks,

- Juri

> > if (!(cpufreq_driver->flags & CPUFREQ_STICKY) &&
> > list_empty(&cpufreq_policy_list)) {
> > /* if all ->init() calls failed, unregister */
> > --
> > 2.2.2
>
> --
> viresh
>

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