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    SubjectRe: [PATCH RFC 3/5] sched/cpufreq: Fix incorrect RCU API usage
    On Thu, Feb 21, 2019 at 10:21:39AM -0500, Joel Fernandes wrote:
    > On Thu, Feb 21, 2019 at 10:18:05AM +0100, Peter Zijlstra wrote:
    > > On Thu, Feb 21, 2019 at 12:49:40AM -0500, Joel Fernandes (Google) wrote:
    > > > @@ -34,8 +34,12 @@ void cpufreq_add_update_util_hook(int cpu, struct update_util_data *data,
    > > > if (WARN_ON(!data || !func))
    > > > return;
    > > >
    > > > - if (WARN_ON(per_cpu(cpufreq_update_util_data, cpu)))
    > > > + rcu_read_lock();
    > > > + if (WARN_ON(rcu_dereference(per_cpu(cpufreq_update_util_data, cpu)))) {
    > > > + rcu_read_unlock();
    > > > return;
    > > > + }
    > > > + rcu_read_unlock();
    > > >
    > > > data->func = func;
    > > > rcu_assign_pointer(per_cpu(cpufreq_update_util_data, cpu), data);
    > >
    > > This doesn't make any kind of sense to me.
    > >
    >
    > As per the rcu_assign_pointer() line, I inferred that
    > cpufreq_update_util_data is expected to be RCU protected. Reading the pointer
    > value of RCU pointers generally needs to be done from RCU read section, and
    > using rcu_dereference() (or using rcu_access()).
    >
    > In this patch, I changed cpufreq_update_util_data to be __rcu annotated to
    > avoid the sparse error thrown by rcu_assign_pointer().
    >
    > Instead of doing that, If your intention here is RELEASE barrier, should I
    > just replace in this function:
    > rcu_assign_pointer(per_cpu(cpufreq_update_util_data, cpu), data);
    > with:
    > smp_store_release(per_cpu(cpufreq_update_util_data, cpu), data))
    > ?
    >
    > It would be nice IMO to be explicit about the intention of release/publish
    > semantics by using smp_store_release().

    No, it is RCU managed, it should be RCU. The problem is that the hunk
    above is utter crap.

    All that does is read the pointer, it never actually dereferences it.

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