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

[Mathieu Desnoyers <mathieu.desnoyers-scC8bbJcJLCw5LPnMra/2Q@public.gmane.org>]

* venkatesh.pallipadi-ral2JQCrhuEAvxtiuMwx3w@public.gmane.org (venkatesh.pallipadi-ral2JQCrhuEAvxtiuMwx3w@public.gmane.org) 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-ral2JQCrhuEAvxtiuMwx3w@public.gmane.org>
> ---
>  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

> +
> +		mutex_lock(&dbs_mutex);
>  		sysfs_remove_group(&policy->kobj, &dbs_attr_group);
>  		dbs_enable--;
>  
> diff --git a/drivers/cpufreq/cpufreq_ondemand.c b/drivers/cpufreq/cpufreq_ondemand.c
> index 1911d17..246ae14 100644
> --- a/drivers/cpufreq/cpufreq_ondemand.c
> +++ b/drivers/cpufreq/cpufreq_ondemand.c
> @@ -78,15 +78,10 @@ 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);
>  
> @@ -494,11 +489,10 @@ static void do_dbs_timer(struct work_struct *work)
>  
>  	delay -= jiffies % delay;
>  
> -	if (lock_policy_rwsem_write(cpu) < 0)
> -		return;
> +	mutex_lock(&dbs_mutex);
>  
>  	if (!dbs_info->enable) {
> -		unlock_policy_rwsem_write(cpu);
> +		mutex_unlock(&dbs_mutex);
>  		return;
>  	}
>  
> @@ -517,7 +511,7 @@ static void do_dbs_timer(struct work_struct *work)
>  			dbs_info->freq_lo, CPUFREQ_RELATION_H);
>  	}
>  	queue_delayed_work_on(cpu, kondemand_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)
> @@ -598,14 +592,15 @@ static int cpufreq_governor_dbs(struct cpufreq_policy *policy,
>  				max(min_sampling_rate,
>  				    latency * LATENCY_MULTIPLIER);
>  		}
> -		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);
> +
> +		mutex_lock(&dbs_mutex);
>  		sysfs_remove_group(&policy->kobj, &dbs_attr_group);
>  		dbs_enable--;
>  		mutex_unlock(&dbs_mutex);
> -- 
> 1.6.0.6
> 
> -- 
> 

-- 
Mathieu Desnoyers
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