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SubjectRe: [PATCH v3 2/2] cpufreq: Convert the cpufreq_driver_lock to use the rcu
On 02/20/2013 11:50 PM, Viresh Kumar wrote:
> 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.
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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