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SubjectRE: [patch 1/4] cpufreq: Eliminate the recent lockdep warnings in cpufreq


>-----Original Message-----
>From: Mathieu Desnoyers [mailto:mathieu.desnoyers@polymtl.ca]
>Sent: Thursday, July 02, 2009 6:07 PM
>To: Pallipadi, Venkatesh
>Cc: Dave Jones; linux-kernel@vger.kernel.org;
>cpufreq@vger.kernel.org; kernel-testers@vger.kernel.org; Ingo
>Molnar; Rafael J. Wysocki; Dave Young; Pekka Enberg; Thomas Renninger
>Subject: Re: [patch 1/4] cpufreq: Eliminate the recent lockdep
>warnings in cpufreq
>
>* venkatesh.pallipadi@intel.com (venkatesh.pallipadi@intel.com) wrote:
>> Commit b14893a62c73af0eca414cfed505b8c09efc613c although it was very
>> much needed to properly cleanup ondemand timer, opened-up a
>can of worms
>> related to locking dependencies in cpufreq.
>>
>> Patch here defines the need for dbs_mutex and cleans up its usage in
>> ondemand governor. This also resolves the lockdep warnings
>reported here
>>
>> http://lkml.indiana.edu/hypermail/linux/kernel/0906.1/01925.html
>> http://lkml.indiana.edu/hypermail/linux/kernel/0907.0/00820.html
>>
>> and few others..
>>
>> Signed-off-by: Venkatesh Pallipadi <venkatesh.pallipadi@intel.com>
>> ---
>> drivers/cpufreq/cpufreq.c | 4 ++--
>> drivers/cpufreq/cpufreq_conservative.c | 27
>+++++++++++----------------
>> drivers/cpufreq/cpufreq_ondemand.c | 27
>+++++++++++----------------
>> 3 files changed, 24 insertions(+), 34 deletions(-)
>>
>> diff --git a/drivers/cpufreq/cpufreq.c b/drivers/cpufreq/cpufreq.c
>> index 6e2ec0b..c7fe16e 100644
>> --- a/drivers/cpufreq/cpufreq.c
>> +++ b/drivers/cpufreq/cpufreq.c
>> @@ -1070,8 +1070,6 @@ static int __cpufreq_remove_dev(struct
>sys_device *sys_dev)
>> spin_unlock_irqrestore(&cpufreq_driver_lock, flags);
>> #endif
>>
>> - unlock_policy_rwsem_write(cpu);
>> -
>> if (cpufreq_driver->target)
>> __cpufreq_governor(data, CPUFREQ_GOV_STOP);
>>
>> @@ -1088,6 +1086,8 @@ static int __cpufreq_remove_dev(struct
>sys_device *sys_dev)
>> if (cpufreq_driver->exit)
>> cpufreq_driver->exit(data);
>>
>> + unlock_policy_rwsem_write(cpu);
>> +
>> free_cpumask_var(data->related_cpus);
>> free_cpumask_var(data->cpus);
>> kfree(data);
>> diff --git a/drivers/cpufreq/cpufreq_conservative.c
>b/drivers/cpufreq/cpufreq_conservative.c
>> index 7fc58af..58889f2 100644
>> --- a/drivers/cpufreq/cpufreq_conservative.c
>> +++ b/drivers/cpufreq/cpufreq_conservative.c
>> @@ -70,15 +70,10 @@ static DEFINE_PER_CPU(struct
>cpu_dbs_info_s, cpu_dbs_info);
>> static unsigned int dbs_enable; /* number of CPUs using
>this policy */
>>
>> /*
>> - * DEADLOCK ALERT! There is a ordering requirement between
>cpu_hotplug
>> - * lock and dbs_mutex. cpu_hotplug lock should always be held before
>> - * dbs_mutex. If any function that can potentially take
>cpu_hotplug lock
>> - * (like __cpufreq_driver_target()) is being called with
>dbs_mutex taken, then
>> - * cpu_hotplug lock should be taken before that. Note that
>cpu_hotplug lock
>> - * is recursive for the same process. -Venki
>> - * DEADLOCK ALERT! (2) : do_dbs_timer() must not take the
>dbs_mutex, because it
>> - * would deadlock with cancel_delayed_work_sync(), which is
>needed for proper
>> - * raceless workqueue teardown.
>> + * dbs_mutex protects data in dbs_tuners_ins from
>concurrent changes on
>> + * different CPUs. It protects dbs_enable in governor
>start/stop. It also
>> + * serializes governor limit_change with do_dbs_timer. We
>do not want
>> + * do_dbs_timer to run when user is changing the governor or limits.
>> */
>> static DEFINE_MUTEX(dbs_mutex);
>>
>> @@ -488,18 +483,17 @@ static void do_dbs_timer(struct
>work_struct *work)
>>
>> delay -= jiffies % delay;
>>
>> - if (lock_policy_rwsem_write(cpu) < 0)
>> - return;
>> + mutex_lock(&dbs_mutex);
>
>OK, I now have absolutely no idea what the rwsem mutex is protecting
>anymore.
>
>You should probably describe the new world order not just in terms of
>what the dbs_mutex is protecting, but also about what the rwsem is
>doing. I'm worried that this rwsem is there to protect against
>more than
>what is protected by the dbs_mutex local to the ondemand/conservative
>governors.
>
>See below,
>
>>
>> if (!dbs_info->enable) {
>> - unlock_policy_rwsem_write(cpu);
>> + mutex_unlock(&dbs_mutex);
>> return;
>> }
>>
>> dbs_check_cpu(dbs_info);
>>
>> queue_delayed_work_on(cpu, kconservative_wq,
>&dbs_info->work, delay);
>> - unlock_policy_rwsem_write(cpu);
>> + mutex_unlock(&dbs_mutex);
>> }
>>
>> static inline void dbs_timer_init(struct cpu_dbs_info_s *dbs_info)
>> @@ -590,15 +584,16 @@ static int cpufreq_governor_dbs(struct
>cpufreq_policy *policy,
>> &dbs_cpufreq_notifier_block,
>> CPUFREQ_TRANSITION_NOTIFIER);
>> }
>> - dbs_timer_init(this_dbs_info);
>> -
>> mutex_unlock(&dbs_mutex);
>>
>> + dbs_timer_init(this_dbs_info);
>> +
>> break;
>>
>> case CPUFREQ_GOV_STOP:
>> - mutex_lock(&dbs_mutex);
>> dbs_timer_exit(this_dbs_info);
>
>So now the only thing that seems to prevent the init and exit to race
>with each other is the rwsem. But this does not seem to be described
>anywhere.

Mathieu,

Yes. rwsem in cpufreq core makes sure that START and STOP happen sequentially. There
Is no way for START and STOP for a CPU to happen at the same time as cpufreq core holds
per policy rwsem lock before making any change to the policy. I can add a comment to
that effect in cpufreq.c. This is a clean seperation across cpufreq core and governor,
as cpufreq core takes care of all the policy changes. With that, do you see any
Issues/races with this patchset?

Thanks,
Venki

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