Re: [PATCH 2/2] cpufreq: Fix a circular lock dependency problem
On Mon, Jul 23, 2018 at 9:27 PM, Waiman Long 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.
Re: [PATCH 2/2] cpufreq: Fix a circular lock dependency problem
On Mon, Jul 23, 2018 at 9:27 PM, Waiman Long 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.
Re: [PATCH 2/2] cpufreq: Fix a circular lock dependency problem
On Mon, Jul 23, 2018 at 9:16 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.
Agreed.
Re: [PATCH 2/2] cpufreq: Fix a circular lock dependency problem
On Mon, Jul 23, 2018 at 9:16 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.
Agreed.
Re: [PATCH 2/2] cpufreq: Fix a circular lock dependency problem
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.
Cheers,
Longman
Re: [PATCH 2/2] cpufreq: Fix a circular lock dependency problem
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.
Cheers,
Longman
Re: [PATCH 2/2] cpufreq: Fix a circular lock dependency problem
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.
Re: [PATCH 2/2] cpufreq: Fix a circular lock dependency problem
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.
