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SubjectRe: [PATCH 2/2] cpufreq: Fix a circular lock dependency problem
On Mon, Jul 23, 2018 at 9:27 PM, Waiman Long <longman@redhat.com> 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.

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