Re: [patch 2/3] cpufreq: Define dbs_mutex purpose and cleanup its usage

Mathieu Desnoyers
2009-06-25T19:48:40Z

* venkatesh.pallipadi@intel.com (venkatesh.pallipadi@intel.com) wrote:
> Commit b14893a62c73af0eca414cfed505b8c09efc613c although it was very
> much needed to cleanup ondemand timer cleanly, openup 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
>
> Signed-off-by: Venkatesh Pallipadi <venkatesh.pallipadi@intel.com>
> ---
> drivers/cpufreq/cpufreq_ondemand.c | 37 +++++++++++++++--------------------
> 1 files changed, 16 insertions(+), 21 deletions(-)
>
> diff --git a/drivers/cpufreq/cpufreq_ondemand.c b/drivers/cpufreq/cpufreq_ondemand.c
> index 1911d17..b2d2106 100644
> --- a/drivers/cpufreq/cpufreq_ondemand.c
> +++ b/drivers/cpufreq/cpufreq_ondemand.c
> @@ -78,15 +78,14 @@ 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 also serializes dbs_enable usage in CPUFREQ_GOV_START
> + * and CPUFREQ_GOV_STOP.
> + *
> + * dbs_mutex should be always held after lock_policy_rwsem whenever needed.
> + * 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.
> */
> static DEFINE_MUTEX(dbs_mutex);
>
> @@ -240,12 +239,10 @@ static ssize_t store_sampling_rate(struct cpufreq_policy *unused,
> unsigned int input;
> int ret;
> ret = sscanf(buf, "%u", &input);
> + if (ret != 1)
> + return -EINVAL;
>
> mutex_lock(&dbs_mutex);
> - if (ret != 1) {
> - mutex_unlock(&dbs_mutex);
> - return -EINVAL;
> - }
> dbs_tuners_ins.sampling_rate = max(input, min_sampling_rate);
> mutex_unlock(&dbs_mutex);
>
> @@ -258,14 +255,12 @@ static ssize_t store_up_threshold(struct cpufreq_policy *unused,
> unsigned int input;
> int ret;
> ret = sscanf(buf, "%u", &input);
> -
> - mutex_lock(&dbs_mutex);
> if (ret != 1 || input > MAX_FREQUENCY_UP_THRESHOLD ||
> input < MIN_FREQUENCY_UP_THRESHOLD) {
> - mutex_unlock(&dbs_mutex);
> return -EINVAL;
> }
>
> + mutex_lock(&dbs_mutex);
> dbs_tuners_ins.up_threshold = input;
> mutex_unlock(&dbs_mutex);
>
> @@ -324,8 +319,8 @@ static ssize_t store_powersave_bias(struct cpufreq_policy *unused,
>
> mutex_lock(&dbs_mutex);
> dbs_tuners_ins.powersave_bias = input;
> - ondemand_powersave_bias_init();
> mutex_unlock(&dbs_mutex);
> + ondemand_powersave_bias_init();
>
> return count;
> }
> @@ -598,14 +593,16 @@ static int cpufreq_governor_dbs(struct cpufreq_policy *policy,
> max(min_sampling_rate,
> latency * LATENCY_MULTIPLIER);
> }
> + mutex_unlock(&dbs_mutex);
> +
> dbs_timer_init(this_dbs_info);
>
> - mutex_unlock(&dbs_mutex);
> break;
>
> case CPUFREQ_GOV_STOP:
> - mutex_lock(&dbs_mutex);
> dbs_timer_exit(this_dbs_info);

Hrm, so.. how do we protect against concurrent :

CPUFREQ_GOV_START/CPUFREQ_GOV_STOP now ?

Mathieu

> +
> + mutex_lock(&dbs_mutex);
> sysfs_remove_group(&policy->kobj, &dbs_attr_group);
> dbs_enable--;
> mutex_unlock(&dbs_mutex);
> @@ -613,14 +610,12 @@ static int cpufreq_governor_dbs(struct cpufreq_policy *policy,
> break;
>
> case CPUFREQ_GOV_LIMITS:
> - mutex_lock(&dbs_mutex);
> if (policy->max < this_dbs_info->cur_policy->cur)
> __cpufreq_driver_target(this_dbs_info->cur_policy,
> policy->max, CPUFREQ_RELATION_H);
> else if (policy->min > this_dbs_info->cur_policy->cur)
> __cpufreq_driver_target(this_dbs_info->cur_policy,
> policy->min, CPUFREQ_RELATION_L);
> - mutex_unlock(&dbs_mutex);
> break;
> }
> return 0;
> --
> 1.6.0.6
>
> --
>

--
Mathieu Desnoyers
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