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Date: Mon, 20 May 2013 14:39:22 +0530
Message-ID: <CAKohponS+tCkZyVpDO9fEMQCfsn5h=N235sj5sBGUkD2qKY=cQ@mail.gmail.com>
Subject: Re: NOHZ: WARNING: at arch/x86/kernel/smp.c:123 native_smp_send_reschedule,
 round 2
From: Viresh Kumar <viresh.kumar@linaro.org>
To: Michael Wang <wangyun@linux.vnet.ibm.com>
Cc: Borislav Petkov <bp@alien8.de>, Tejun Heo <tj@kernel.org>,
        "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
        Jiri Kosina <jkosina@suse.cz>,
        Frederic Weisbecker <fweisbec@gmail.com>,
        Tony Luck <tony.luck@intel.com>, linux-kernel@vger.kernel.org,
        x86@kernel.org, Thomas Gleixner <tglx@linutronix.de>, rjw@sisk.pl,
        cpufreq@vger.kernel.org, linux-pm@vger.kernel.org
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On 20 May 2013 14:26, Michael Wang <wangyun@linux.vnet.ibm.com> wrote:
> On 05/20/2013 03:25 PM, Michael Wang wrote:
>> Yeah, that's right, I guess the issue is, although the policy->cpus is
>> correct at a given time, after get cpu from it, it's possible to be
>> changed, unless we disabled preempt or irq, or hotplug before we use it...
>>
>> Like such issue cases:
>>                               get x from policy->cpus
>>       DOWN notifier
>>       change policy->cpus
>>       do offline x
>>                               send ipi to x
>>
>> Will that happen?

Sorry I am not sure. :(

I can see mutex being used in cpufreq_governor.c which should take care
of race conditions...

> May be we could do some test to confirm it?
>
> diff --git a/drivers/cpufreq/cpufreq_governor.c
> b/drivers/cpufreq/cpufreq_governor.c
> index 443442d..449be88 100644
> --- a/drivers/cpufreq/cpufreq_governor.c
> +++ b/drivers/cpufreq/cpufreq_governor.c
> @@ -26,6 +26,7 @@
>  #include <linux/tick.h>
>  #include <linux/types.h>
>  #include <linux/workqueue.h>
> +#include <linux/cpu.h>
>
>  #include "cpufreq_governor.h"
>
> @@ -180,8 +181,10 @@ void gov_queue_work(struct dbs_data *dbs_data,
> struct cpufreq_policy *policy,
>         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);
>
> This is supposed to make WARN disappear, if it works, then BINGO :)

Let people test it and then we can talk :)
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