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Date: Wed, 10 Jul 2013 16:57:35 +0800
From: Michael Wang <wangyun@...ux.vnet.ibm.com>
To: Bartlomiej Zolnierkiewicz <b.zolnierkie@...sung.com>
CC: "Rafael J. Wysocki" <rafael.j.wysocki@...el.com>,
Viresh Kumar <viresh.kumar@...aro.org>,
Borislav Petkov <bp@...en8.de>, Jiri Kosina <jkosina@...e.cz>,
Tomasz Figa <t.figa@...sung.com>, linux-kernel@...r.kernel.org,
linux-pm@...r.kernel.org
Subject: Re: [v3.10 regression] deadlock on cpu hotplug
On 07/10/2013 10:40 AM, Michael Wang wrote:
> On 07/09/2013 07:51 PM, Bartlomiej Zolnierkiewicz wrote:
> [snip]
>>
>> It doesn't help and unfortunately it just can't help as it only
>> addresses lockdep functionality while the issue is not a lockdep
>> problem but a genuine locking problem. CPU hot-unplug invokes
>> _cpu_down() which calls cpu_hotplug_begin() which in turn takes
>> &cpu_hotplug.lock. The lock is then hold during __cpu_notify()
>> call. Notifier chain goes up to cpufreq_governor_dbs() which for
>> CPUFREQ_GOV_STOP event does gov_cancel_work(). This function
>> flushes pending work and waits for it to finish. The all above
>> happens in one kernel thread. At the same time the other kernel
>> thread is doing the work we are waiting to complete and it also
>> happens to do gov_queue_work() which calls get_online_cpus().
>> Then the code tries to take &cpu_hotplug.lock which is already
>> held by the first thread and deadlocks.
>
> Hmm...I think I get your point, some thread hold the lock and
> flush some work which also try to hold the same lock, correct?
>
> Ok, that's a problem, let's figure out a good way to solve it :)
And besides the patch from Srivatsa, I also suggest below fix, it's
try to really stop all the works during down notify, I'd like to know
how do you think about it ;-)
Regards,
Michael Wang
diff --git a/drivers/cpufreq/cpufreq_governor.c b/drivers/cpufreq/cpufreq_governor.c
index dc9b72e..a64b544 100644
--- a/drivers/cpufreq/cpufreq_governor.c
+++ b/drivers/cpufreq/cpufreq_governor.c
@@ -178,13 +178,14 @@ void gov_queue_work(struct dbs_data *dbs_data, struct cpufreq_policy *policy,
{
int i;
+ if (dbs_data->queue_stop)
+ return;
+
if (!all_cpus) {
__gov_queue_work(smp_processor_id(), dbs_data, delay);
} else {
- get_online_cpus();
for_each_cpu(i, policy->cpus)
__gov_queue_work(i, dbs_data, delay);
- put_online_cpus();
}
}
EXPORT_SYMBOL_GPL(gov_queue_work);
@@ -193,12 +194,27 @@ static inline void gov_cancel_work(struct dbs_data *dbs_data,
struct cpufreq_policy *policy)
{
struct cpu_dbs_common_info *cdbs;
- int i;
+ int i, round = 2;
+ dbs_data->queue_stop = 1;
+redo:
+ round--;
for_each_cpu(i, policy->cpus) {
cdbs = dbs_data->cdata->get_cpu_cdbs(i);
cancel_delayed_work_sync(&cdbs->work);
}
+
+ /*
+ * Since there is no lock to prvent re-queue the
+ * cancelled work, some early cancelled work might
+ * have been queued again by later cancelled work.
+ *
+ * Flush the work again with dbs_data->queue_stop
+ * enabled, this time there will be no survivors.
+ */
+ if (round)
+ goto redo;
+ dbs_data->queue_stop = 0;
}
/* Will return if we need to evaluate cpu load again or not */
diff --git a/drivers/cpufreq/cpufreq_governor.h b/drivers/cpufreq/cpufreq_governor.h
index e16a961..9116135 100644
--- a/drivers/cpufreq/cpufreq_governor.h
+++ b/drivers/cpufreq/cpufreq_governor.h
@@ -213,6 +213,7 @@ struct dbs_data {
unsigned int min_sampling_rate;
int usage_count;
void *tuners;
+ int queue_stop;
/* dbs_mutex protects dbs_enable in governor start/stop */
struct mutex mutex;
>
> Regards,
> Michael Wang
>
>
>
>
>>
>> Best regards,
>> --
>> Bartlomiej Zolnierkiewicz
>> Samsung R&D Institute Poland
>> Samsung Electronics
>>
>>> diff --git a/drivers/cpufreq/cpufreq_governor.c b/drivers/cpufreq/cpufreq_governor.c
>>> index 5af40ad..aa05eaa 100644
>>> --- a/drivers/cpufreq/cpufreq_governor.c
>>> +++ b/drivers/cpufreq/cpufreq_governor.c
>>> @@ -229,6 +229,8 @@ static void set_sampling_rate(struct dbs_data *dbs_data,
>>> }
>>> }
>>>
>>> +static struct lock_class_key j_cdbs_key;
>>> +
>>> int cpufreq_governor_dbs(struct cpufreq_policy *policy,
>>> struct common_dbs_data *cdata, unsigned int event)
>>> {
>>> @@ -366,6 +368,8 @@ int (struct cpufreq_policy *policy,
>>> kcpustat_cpu(j).cpustat[CPUTIME_NICE];
>>>
>>> mutex_init(&j_cdbs->timer_mutex);
>>> + lockdep_set_class(&j_cdbs->timer_mutex, &j_cdbs_key);
>>> +
>>> INIT_DEFERRABLE_WORK(&j_cdbs->work,
>>> dbs_data->cdata->gov_dbs_timer);
>>> }
>>>
>>> Regards,
>>> Michael Wang
>>
>
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