Re: [PATCH 2/2] cpufreq: Fix a circular lock dependency problem

["Rafael J. Wysocki" <rafael@kernel.org>]

On Mon, Jul 23, 2018 at 9:16 PM, Peter Zijlstra <peterz@infradead.org> wrote:
> On Mon, Jul 23, 2018 at 01:49:39PM -0400, Waiman Long wrote:
>>  drivers/cpufreq/cpufreq.c | 16 +++++++++++++++-
>>  1 file changed, 15 insertions(+), 1 deletion(-)
>>
>> diff --git a/drivers/cpufreq/cpufreq.c b/drivers/cpufreq/cpufreq.c
>> index b0dfd32..9cf02d7 100644
>> --- a/drivers/cpufreq/cpufreq.c
>> +++ b/drivers/cpufreq/cpufreq.c
>> @@ -922,8 +922,22 @@ static ssize_t store(struct kobject *kobj, struct attribute *attr,
>>       struct cpufreq_policy *policy = to_policy(kobj);
>>       struct freq_attr *fattr = to_attr(attr);
>>       ssize_t ret = -EINVAL;
>> +     int retries = 3;
>>
>> -     cpus_read_lock();
>> +     /*
>> +      * cpus_read_trylock() is used here to work around a circular lock
>> +      * dependency problem with respect to the cpufreq_register_driver().
>> +      * With a simple retry loop, the chance of not able to get the
>> +      * read lock is extremely small.
>> +      */
>> +     while (!cpus_read_trylock()) {
>> +             if (retries-- <= 0)
>> +                     return -EBUSY;
>> +             /*
>> +              * Sleep for about 50ms and retry again.
>> +              */
>> +             msleep(50);
>> +     }
>
> That's atrocious.

Agreed.