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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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