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

From: Nathan Zimmer
Date: Thu Feb 21 2013 - 12:49:12 EST

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
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.


if (cpufreq_disabled())

- 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

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.

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