Re: [PATCH v3 2/2] cpufreq: Convert the cpufreq_driver_lock to use the rcu

From: Viresh Kumar <viresh.kumar@linaro.org>
To: Nathan Zimmer <nzimmer@sgi.com>
Cc: rjw@sisk.pl, cpufreq@vger.kernel.org, linux-pm@vger.kernel.org,
	linux-kernel@vger.kernel.org
Subject: Re: [PATCH v3 2/2] cpufreq: Convert the cpufreq_driver_lock to use the rcu
Date: Thu, 21 Feb 2013 11:20:26 +0530	[thread overview]
Message-ID: <CAKohponMWRPspZF=tPN6MXoxkcjw7CK_sLnSdsKe+EDoiADvGA@mail.gmail.com> (raw)
In-Reply-To: <1361404583-5557-3-git-send-email-nzimmer@sgi.com>

On 21 February 2013 05:26, Nathan Zimmer <nzimmer@sgi.com> wrote:
> In general rwlocks are discourged so we are moving it to use the rcu instead.
> This does require a bit of care since the cpufreq_driver_lock protects both
> the cpufreq_driver and the cpufreq_cpu_data array.
> Also since many of the function pointers on cpufreq_driver may sleep when
> called we have to grab them under the rcu_read_lock but call them after
> rcu_read_unlock();

Even i have started reading rcu documentation now :)

> Cc: Viresh Kumar <viresh.kumar@linaro.org>
> Cc: "Rafael J. Wysocki" <rjw@sisk.pl>
> Signed-off-by: Nathan Zimmer <nzimmer@sgi.com>
> ---
>  drivers/cpufreq/cpufreq.c | 312 +++++++++++++++++++++++++++++++++-------------
>  1 file changed, 224 insertions(+), 88 deletions(-)
>
> diff --git a/drivers/cpufreq/cpufreq.c b/drivers/cpufreq/cpufreq.c

> @@ -255,20 +258,21 @@ static inline void adjust_jiffies(unsigned long val, struct cpufreq_freqs *ci)
>  void cpufreq_notify_transition(struct cpufreq_freqs *freqs, unsigned int state)
>  {
>         struct cpufreq_policy *policy;
> -       unsigned long flags;
> +       u8 flags;

I think you can get rid of flags.

>         BUG_ON(irqs_disabled());
>
>         if (cpufreq_disabled())
>                 return;
>
> -       freqs->flags = cpufreq_driver->flags;
>         pr_debug("notification %u of frequency transition to %u kHz\n",
>                 state, freqs->new);
>
> -       read_lock_irqsave(&cpufreq_driver_lock, flags);
> +       rcu_read_lock();
> +       flags = rcu_dereference(cpufreq_driver)->flags;

use freq->flags here ...

>         policy = per_cpu(cpufreq_cpu_data, freqs->cpu);
> -       read_unlock_irqrestore(&cpufreq_driver_lock, flags);
> +       rcu_read_unlock();
> +       freqs->flags = flags;
>
>         switch (state) {
>
> @@ -277,7 +281,7 @@ void cpufreq_notify_transition(struct cpufreq_freqs *freqs, unsigned int state)
>                  * which is not equal to what the cpufreq core thinks is
>                  * "old frequency".
>                  */
> -               if (!(cpufreq_driver->flags & CPUFREQ_CONST_LOOPS)) {
> +               if (!(flags & CPUFREQ_CONST_LOOPS)) {

and here.

>                         if ((policy) && (policy->cpu == freqs->cpu) &&
>                             (policy->cur) && (policy->cur != freqs->old)) {
>                                 pr_debug("Warning: CPU frequency is"

> @@ -742,35 +773,39 @@ static int cpufreq_add_dev_interface(unsigned int cpu,

> -       write_lock_irqsave(&cpufreq_driver_lock, flags);
> +       spin_lock_irqsave(&cpufreq_driver_lock, flags);
>         for_each_cpu(j, policy->cpus) {
>                 per_cpu(cpufreq_cpu_data, j) = policy;
>                 per_cpu(cpufreq_policy_cpu, j) = policy->cpu;
>         }
> -       write_unlock_irqrestore(&cpufreq_driver_lock, flags);
> +       spin_unlock_irqrestore(&cpufreq_driver_lock, flags);
> +       synchronize_rcu();

I don't think (but i can be wrong too :) ), that we need a synchronize_rcu()
here. We need it only at places where we have updated the cpufreq_driver
pointer.

As we aren't doing any rcu specific read/update for cpufreq_cpu_data.