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

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Date:   Mon, 23 Jul 2018 15:27:56 -0400
From:   Waiman Long <longman@...hat.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Viresh Kumar <viresh.kumar@...aro.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...nel.org>, linux-kernel@...r.kernel.org,
        linux-pm@...r.kernel.org,
        "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
Subject: 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