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

[Nathan Zimmer]

On 02/20/2013 11:50 PM, Viresh Kumar wrote:
> On 21 February 2013 05:26, Nathan Zimmer <nzimmer@xxxxxxx> 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@xxxxxxxxxx>
> > Cc: "Rafael J. Wysocki" <rjw@xxxxxxx>
> > Signed-off-by: Nathan Zimmer <nzimmer@xxxxxxx>
> > ---
> >   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.
Of course.
> >                          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.
Good point.
I placed a similar sycnronize_rcu in cpufreq_add_policy_cpu and 
cpufreq_add_dev.
I will remove them also.


Thanks, I will respin.
Nate

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