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

[Rafael J. Wysocki]

On Mon, Jul 23, 2018 at 9:27 PM, Waiman Long <longman@xxxxxxxxxx> wrote:
> On 07/23/2018 03:16 PM, Peter Zijlstra 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.
>>
>>
> I had thought about just returning an error if the trylock fails as CPU
> hotplug rarely happened. I can revert to that simple case if others have
> no objection.

Yes, you can return -EBUSY or -EAGAIN right away from here if the
cpus_read_trylock() is not successful.  There is not much reason for
the sysfs operation to continue in that case.