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

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Message-ID: <CAJZ5v0hbpk4FJQ3m4HCj_X44Jjp1JM21hwAAeisALmJQb8Cyfw@mail.gmail.com>
Date:   Tue, 24 Jul 2018 10:31:35 +0200
From:   "Rafael J. Wysocki" <rafael@...nel.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Waiman Long <longman@...hat.com>,
        "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Viresh Kumar <viresh.kumar@...aro.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...nel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux PM <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 Mon, Jul 23, 2018 at 9:16 PM, Peter Zijlstra <peterz@...radead.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.