On Tuesday, March 04, 2014 12:42:15 PM Aaron Plattner wrote:
> If a module calls cpufreq_get while cpufreq is initializing, it's possible for
> it to be called after cpufreq_driver is set but before cpufreq_cpu_data is
> written during subsys_interface_register. This happens because cpufreq_get
> doesn't take the cpufreq_driver_lock around its use of cpufreq_cpu_data.
Is this a theoretical race, or can you actually reproduce it? If so, on what
system/driver? Or are there any bug reports related to this you can point me
to?
> Fix this by using cpufreq_cpu_get(cpu) to look up the policy rather than reading
> it out of cpufreq_cpu_data directly. cpufreq_cpu_get takes the appropriate
> locks to prevent this race from happening.
>
> Since it's possible for policy to be NULL if the caller passes in an invalid CPU
> number or calls the function before cpufreq is initialized, delete the
> BUG_ON(!policy) and simply return 0. Don't try to return -ENOENT because that's
> negative and the function returns an unsigned integer.
>
> Signed-off-by: Aaron Plattner <aplattner@xxxxxxxxxx>
Viresh, have you seen this?
> ---
> drivers/cpufreq/cpufreq.c | 21 +++++++--------------
> 1 file changed, 7 insertions(+), 14 deletions(-)
>
> diff --git a/drivers/cpufreq/cpufreq.c b/drivers/cpufreq/cpufreq.c
> index 8d19f7c..158d0b5 100644
> --- a/drivers/cpufreq/cpufreq.c
> +++ b/drivers/cpufreq/cpufreq.c
> @@ -1447,23 +1447,16 @@ static unsigned int __cpufreq_get(unsigned int cpu)
> */
> unsigned int cpufreq_get(unsigned int cpu)
> {
> - struct cpufreq_policy *policy = per_cpu(cpufreq_cpu_data, cpu);
> + struct cpufreq_policy *policy = cpufreq_cpu_get(cpu);
> unsigned int ret_freq = 0;
>
> - if (cpufreq_disabled() || !cpufreq_driver)
> - return -ENOENT;
> -
> - BUG_ON(!policy);
> -
> - if (!down_read_trylock(&cpufreq_rwsem))
> - return 0;
> -
> - down_read(&policy->rwsem);
> -
> - ret_freq = __cpufreq_get(cpu);
> + if (policy) {
> + down_read(&policy->rwsem);
> + ret_freq = __cpufreq_get(cpu);
> + up_read(&policy->rwsem);
>
> - up_read(&policy->rwsem);
> - up_read(&cpufreq_rwsem);
> + cpufreq_cpu_put(policy);
> + }
>
> return ret_freq;
> }
>
--
I speak only for myself.
Rafael J. Wysocki, Intel Open Source Technology Center.
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