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Date: Tue, 1 Mar 2016 13:57:27 +0100
From: Peter Zijlstra <peterz@infradead.org>
To: "Rafael J. Wysocki" <rjw@rjwysocki.net>
Cc: Steve Muckle <steve.muckle@linaro.org>, Ingo Molnar <mingo@redhat.com>,
        "Rafael J. Wysocki" <rafael@kernel.org>, linux-kernel@vger.kernel.org,
        linux-pm@vger.kernel.org, Vincent Guittot <vincent.guittot@linaro.org>,
        Morten Rasmussen <morten.rasmussen@arm.com>,
        Dietmar Eggemann <dietmar.eggemann@arm.com>,
        Juri Lelli <Juri.Lelli@arm.com>,
        Patrick Bellasi <patrick.bellasi@arm.com>,
        Michael Turquette <mturquette@baylibre.com>,
        Ricky Liang <jcliang@chromium.org>
Subject: Re: [RFCv7 PATCH 03/10] sched: scheduler-driven cpu frequency
 selection
Message-ID: <20160301125727.GT6357@twins.programming.kicks-ass.net>
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On Sat, Feb 27, 2016 at 01:08:02AM +0100, Rafael J. Wysocki wrote:
> @@ -95,18 +98,20 @@ EXPORT_SYMBOL_GPL(cpufreq_set_update_uti
>   *
>   * This function is called by the scheduler on every invocation of
>   * update_load_avg() on the CPU whose utilization is being updated.
> + *
> + * It can only be called from RCU-sched read-side critical sections.
>   */
>  void cpufreq_update_util(u64 time, unsigned long util, unsigned long max)
>  {
>  	struct update_util_data *data;
>  
> -	rcu_read_lock();
> -

Maybe, just because I'm paranoid, add something like:

#ifdef CONFIG_LOCKDEP
	WARN_ON(debug_locks && !rcu_read_lock_sched_held());
#endif

>  	data = rcu_dereference(*this_cpu_ptr(&cpufreq_update_util_data));
> -	if (data && data->func)
> +	/*
> +	 * If this isn't inside of an RCU-sched read-side critical section, data
> +	 * may become NULL after the check below.
> +	 */
> +	if (data)
>  		data->func(data, time, util, max);
> -
> -	rcu_read_unlock();
>  }