Date:	Wed, 25 Feb 2015 11:24:23 +0800
From:	Ethan Zhao <ethan.kernel@...il.com>
To:	ethan zhao <ethan.zhao@...cle.com>
Cc:	Viresh Kumar <viresh.kumar@...aro.org>,
	Rafael Wysocki <rjw@...ysocki.net>,
	santosh shilimkar <santosh.shilimkar@...cle.com>,
	Linaro Kernel Mailman List <linaro-kernel@...ts.linaro.org>,
	"linux-pm@...r.kernel.org" <linux-pm@...r.kernel.org>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	guangyu.sun@...cle.com,
	"sriharsha.devdas@...cle.com" <sriharsha.devdas@...cle.com>
Subject: Re: [PATCH Resend] cpufreq: Set cpufreq_cpu_data to NULL before
 putting kobject

Viresh,
  With this patch applied, still got the following warning and panic,
seems it needs more care.

     54.474618] ------------[ cut here ]------------
[   54.545816] WARNING: CPU: 0 PID: 213 at include/linux/kref.h:47
kobject_get+0x41/0x50()
[   54.642595] Modules linked in: i2c_i801(+) mfd_core shpchp(+)
acpi_cpufreq(+) edac_core ioatdma(+) xfs libcrc32c ast syscopyarea ixgbe
sysfillrect sysimgblt sr_mod sd_mod drm_kms_helper igb mdio cdrom e1000e ahci
dca ttm libahci uas drm i2c_algo_bit ptp megaraid_sas libata usb_storage
i2c_core pps_core dm_mirror dm_region_hash dm_log dm_mod
[   55.007264] CPU: 0 PID: 213 Comm: kworker/0:2 Not tainted
3.18.5
[   55.099970] Hardware name: Oracle Corporation SUN FIRE X4170 M2 SERVER
   /ASSY,MOTHERBOARD,X4170, BIOS 08120104 05/08/2012
[   55.239736] Workqueue: kacpi_notify acpi_os_execute_deferred
[   55.308598]  0000000000000000 00000000bd730b61 ffff88046742baf8
ffffffff816b7edb
[   55.398305]  0000000000000000 0000000000000000 ffff88046742bb38
ffffffff81078ae1
[   55.488040]  ffff88046742bbd8 ffff8806706b3000 0000000000000292
0000000000000000
[   55.577776] Call Trace:
[   55.608228]  [<ffffffff816b7edb>] dump_stack+0x46/0x58
[   55.670895]  [<ffffffff81078ae1>] warn_slowpath_common+0x81/0xa0
[   55.743952]  [<ffffffff81078bfa>] warn_slowpath_null+0x1a/0x20
[   55.814929]  [<ffffffff8130d0d1>] kobject_get+0x41/0x50
[   55.878654]  [<ffffffff8153e955>] cpufreq_cpu_get+0x75/0xc0
[   55.946528]  [<ffffffff8153f37e>] cpufreq_update_policy+0x2e/0x1f0
[   56.021682]  [<ffffffff810bf9d2>] ? up+0x32/0x50
[   56.078126]  [<ffffffff813ab975>] ? acpi_ns_get_node+0xcb/0xf2
[   56.148974]  [<ffffffff813abdc9>] ? acpi_evaluate_object+0x22c/0x252
[   56.226066]  [<ffffffff813ac3c2>] ? acpi_get_handle+0x95/0xc0
[   56.295871]  [<ffffffff8138a7fb>] ? acpi_has_method+0x25/0x40
[   56.365661]  [<ffffffff813bbcd4>] acpi_processor_ppc_has_changed+0x77/0x82
[   56.448956]  [<ffffffff8108f726>] ? move_linked_works+0x66/0x90
[   56.520842]  [<ffffffff813b87b9>] acpi_processor_notify+0x58/0xe7
[   56.594807]  [<ffffffff8139dfd8>] acpi_ev_notify_dispatch+0x44/0x5c
[   56.670859]  [<ffffffff81388fe3>] acpi_os_execute_deferred+0x15/0x22
[   56.747936]  [<ffffffff8109268e>] process_one_work+0x14e/0x3f0
[   56.818766]  [<ffffffff81092d9b>] worker_thread+0x11b/0x4d0
[   56.886486]  [<ffffffff81092c80>] ? rescuer_thread+0x350/0x350
[   56.957316]  [<ffffffff810984f1>] kthread+0xe1/0x100
[   57.017742]  [<ffffffff81098410>] ? kthread_create_on_node+0x1b0/0x1b0
[   57.096903]  [<ffffffff816bfe7c>] ret_from_fork+0x7c/0xb0
[   57.162534]  [<ffffffff81098410>] ? kthread_create_on_node+0x1b0/0x1b0
[   57.241680] ---[ end trace dce06bb76f547de5 ]---


Any idea ?

Thanks,
Ethan

On Mon, Feb 2, 2015 at 11:04 PM, ethan zhao <ethan.zhao@...cle.com> wrote:
>
> On 2015/2/2 12:54, Viresh Kumar wrote:
>>
>> On 2 February 2015 at 10:15, ethan zhao <ethan.zhao@...cle.com> wrote:
>>>
>>> On 2015/2/2 12:26, Viresh Kumar wrote:
>>>   But there is no checking against refcount in or before
>>>
>>>   cpufreq_policy_free(), that is one issue I mentioned.
>>
>> As I said earlier, the completion will only fire once the refcount
>> is zero. And so there is no need of any check here.
>>
>>>> That routines doesn't have any tricks and simply frees the policy.
>>>> Because, before calling cpufreq_policy_put_kobj(), we have set
>>>> the per-cpu variable to NULL, nobody else will get the policy
>>>
>>>   It is possible cpufreq_cpu_get() within the PPC thread was called just
>>>   before __cpufreq_remove_dev_finish() is to be called in another thread,
>>>   so you set the per_cpu(cpufreq_cpu_data, cpu) to NULL will not prevent
>>>   the actions between cpufreq_cpu_get and cpufreq_cpu_put().
>>>
>>>   And then the freeing happens in __cpufreq_remove_dev_finish().
>>
>> It will.. You aren't looking closely enough. If cpufreq_cpu_get() is
>> called just
>> before remove-dev, then cpufreq_cpu_get() will take:
>>
>> read_lock_irqsave(&cpufreq_driver_lock, flags);
>>
>> And it will do:
>>
>> read_unlock_irqrestore(&cpufreq_driver_lock, flags);
>>
>> only after increasing the refcount with kobject_get().
>>
>> While on the other side __cpufreq_remove_dev_finish() will do this:
>>
>>         write_lock_irqsave(&cpufreq_driver_lock, flags);
>>         policy = per_cpu(cpufreq_cpu_data, cpu);
>>         per_cpu(cpufreq_cpu_data, cpu) = NULL;
>>         write_unlock_irqrestore(&cpufreq_driver_lock, flags);
>>
>> So, it will wait for the read_lock in cpufreq_cpu_get() to finish before
>> setting per-cpu variable to NULL. And so, after kobject_put() in
>> cpufreq_policy_put_kobj(), we will wait for the completion to fire
>
>  Closely enough this time, understood, thanks for your explanation.
>
>
>  Ethan
>
>> and that will only happen once a corresponding cpufreq_cpu_put()
>> is issued.
>
>
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