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cpufreq: governor: Create and traverse list of policy_dbs to avoid deadlock

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The dbs_data_mutex lock is currently used in two places.  First,
cpufreq_governor_dbs() uses it to guarantee mutual exclusion between
invocations of governor operations from the core.  Second, it is used by
ondemand governor's update_sampling_rate() to ensure the stability of
data structures walked by it.

The second usage is quite problematic, because update_sampling_rate() is
called from a governor sysfs attribute's ->store callback and that leads
to a deadlock scenario involving cpufreq_governor_exit() which runs
under dbs_data_mutex.  Thus it is better to rework the code so
update_sampling_rate() doesn't need to acquire dbs_data_mutex.

To that end, rework update_sampling_rate() to walk a list of policy_dbs
objects supported by the dbs_data one it has been called for (instead of
walking cpu_dbs_info object for all CPUs).  The list manipulation is
protected with dbs_data->mutex which also is held around the execution
of update_sampling_rate(), it is not necessary to hold dbs_data_mutex in
that function any more.

Reported-by: Juri Lelli <juri.lelli@arm.com>
Reported-by: Shilpasri G Bhat <shilpa.bhat@linux.vnet.ibm.com>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
[ rjw: Subject & changelog ]
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
This commit is contained in:
Viresh Kumar 2016-02-10 11:00:25 +05:30 committed by Rafael J. Wysocki
parent 68e80dae09
commit c54df07184
3 changed files with 54 additions and 64 deletions
Show stats Download patch file Download diff file Expand all files Collapse all files

22
drivers/cpufreq/cpufreq_governor.c
View file

@ -385,9 +385,14 @@ static int cpufreq_governor_init(struct cpufreq_policy *policy)
ret = -EINVAL;
goto free_policy_dbs_info;
}
dbs_data->usage_count++;
policy_dbs->dbs_data = dbs_data;
policy->governor_data = policy_dbs;
mutex_lock(&dbs_data->mutex);
dbs_data->usage_count++;
list_add(&policy_dbs->list, &dbs_data->policy_dbs_list);
mutex_unlock(&dbs_data->mutex);
return 0;
}
@ -397,7 +402,7 @@ static int cpufreq_governor_init(struct cpufreq_policy *policy)
goto free_policy_dbs_info;
}
dbs_data->usage_count = 1;
INIT_LIST_HEAD(&dbs_data->policy_dbs_list);
mutex_init(&dbs_data->mutex);
ret = gov->init(dbs_data, !policy->governor->initialized);
@ -418,9 +423,12 @@ static int cpufreq_governor_init(struct cpufreq_policy *policy)
if (!have_governor_per_policy())
gov->gdbs_data = dbs_data;
policy_dbs->dbs_data = dbs_data;
policy->governor_data = policy_dbs;
policy_dbs->dbs_data = dbs_data;
dbs_data->usage_count = 1;
list_add(&policy_dbs->list, &dbs_data->policy_dbs_list);
gov->kobj_type.sysfs_ops = &governor_sysfs_ops;
ret = kobject_init_and_add(&dbs_data->kobj, &gov->kobj_type,
get_governor_parent_kobj(policy),
@ -448,12 +456,18 @@ static int cpufreq_governor_exit(struct cpufreq_policy *policy)
struct dbs_governor *gov = dbs_governor_of(policy);
struct policy_dbs_info *policy_dbs = policy->governor_data;
struct dbs_data *dbs_data = policy_dbs->dbs_data;
int count;
/* State should be equivalent to INIT */
if (policy_dbs->policy)
return -EBUSY;
if (!--dbs_data->usage_count) {
mutex_lock(&dbs_data->mutex);
list_del(&policy_dbs->list);
count = --dbs_data->usage_count;
mutex_unlock(&dbs_data->mutex);
if (!count) {
kobject_put(&dbs_data->kobj);
policy->governor_data = NULL;

7
drivers/cpufreq/cpufreq_governor.h
View file

@ -73,7 +73,11 @@ struct dbs_data {
unsigned int up_threshold;
struct kobject kobj;
/* Protect concurrent updates to governor tunables from sysfs */
struct list_head policy_dbs_list;
/*
* Protect concurrent updates to governor tunables from sysfs,
* policy_dbs_list and usage_count.
*/
struct mutex mutex;
};
@ -125,6 +129,7 @@ struct policy_dbs_info {
struct work_struct work;
/* dbs_data may be shared between multiple policy objects */
struct dbs_data *dbs_data;
struct list_head list;
};
static inline void gov_update_sample_delay(struct policy_dbs_info *policy_dbs,

89
drivers/cpufreq/cpufreq_ondemand.c
View file

@ -226,84 +226,55 @@ static struct dbs_governor od_dbs_gov;
* @new_rate: new sampling rate
*
* If new rate is smaller than the old, simply updating
* dbs_tuners_int.sampling_rate might not be appropriate. For example, if the
* dbs.sampling_rate might not be appropriate. For example, if the
* original sampling_rate was 1 second and the requested new sampling rate is 10
* ms because the user needs immediate reaction from ondemand governor, but not
* sure if higher frequency will be required or not, then, the governor may
* change the sampling rate too late; up to 1 second later. Thus, if we are
* reducing the sampling rate, we need to make the new value effective
* immediately.
*
* On the other hand, if new rate is larger than the old, then we may evaluate
* the load too soon, and it might we worth updating sample_delay_ns then as
* well.
*
* This must be called with dbs_data->mutex held, otherwise traversing
* policy_dbs_list isn't safe.
*/
static void update_sampling_rate(struct dbs_data *dbs_data,
unsigned int new_rate)
{
struct cpumask cpumask;
int cpu;
struct policy_dbs_info *policy_dbs;
dbs_data->sampling_rate = new_rate = max(new_rate,
dbs_data->min_sampling_rate);
/*
* Lock governor so that governor start/stop can't execute in parallel.
* We are operating under dbs_data->mutex and so the list and its
* entries can't be freed concurrently.
*/
mutex_lock(&dbs_data_mutex);
cpumask_copy(&cpumask, cpu_online_mask);
for_each_cpu(cpu, &cpumask) {
struct cpufreq_policy *policy;
struct od_cpu_dbs_info_s *dbs_info;
struct cpu_dbs_info *cdbs;
struct policy_dbs_info *policy_dbs;
dbs_info = &per_cpu(od_cpu_dbs_info, cpu);
cdbs = &dbs_info->cdbs;
policy_dbs = cdbs->policy_dbs;
list_for_each_entry(policy_dbs, &dbs_data->policy_dbs_list, list) {
mutex_lock(&policy_dbs->timer_mutex);
/*
* A valid policy_dbs and policy_dbs->policy means governor
* hasn't stopped or exited yet.
* On 32-bit architectures this may race with the
* sample_delay_ns read in dbs_update_util_handler(), but that
* really doesn't matter. If the read returns a value that's
* too big, the sample will be skipped, but the next invocation
* of dbs_update_util_handler() (when the update has been
* completed) will take a sample. If the returned value is too
* small, the sample will be taken immediately, but that isn't a
* problem, as we want the new rate to take effect immediately
* anyway.
*
* If this runs in parallel with dbs_work_handler(), we may end
* up overwriting the sample_delay_ns value that it has just
* written, but the difference should not be too big and it will
* be corrected next time a sample is taken, so it shouldn't be
* significant.
*/
if (!policy_dbs || !policy_dbs->policy)
continue;
policy = policy_dbs->policy;
/* clear all CPUs of this policy */
cpumask_andnot(&cpumask, &cpumask, policy->cpus);
/*
* Update sampling rate for CPUs whose policy is governed by
* dbs_data. In case of governor_per_policy, only a single
* policy will be governed by dbs_data, otherwise there can be
* multiple policies that are governed by the same dbs_data.
*/
if (dbs_data == policy_dbs->dbs_data) {
mutex_lock(&policy_dbs->timer_mutex);
/*
* On 32-bit architectures this may race with the
* sample_delay_ns read in dbs_update_util_handler(),
* but that really doesn't matter. If the read returns
* a value that's too big, the sample will be skipped,
* but the next invocation of dbs_update_util_handler()
* (when the update has been completed) will take a
* sample. If the returned value is too small, the
* sample will be taken immediately, but that isn't a
* problem, as we want the new rate to take effect
* immediately anyway.
*
* If this runs in parallel with dbs_work_handler(), we
* may end up overwriting the sample_delay_ns value that
* it has just written, but the difference should not be
* too big and it will be corrected next time a sample
* is taken, so it shouldn't be significant.
*/
gov_update_sample_delay(policy_dbs, new_rate);
mutex_unlock(&policy_dbs->timer_mutex);
}
gov_update_sample_delay(policy_dbs, new_rate);
mutex_unlock(&policy_dbs->timer_mutex);
}
mutex_unlock(&dbs_data_mutex);
}
static ssize_t store_sampling_rate(struct dbs_data *dbs_data, const char *buf,