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Subject: Re: [PATCH v1] PM-runtime: Check supplier_preactivated before release
 supplier
To:     "Rafael J. Wysocki" <rafael@kernel.org>
CC:     "Rafael J. Wysocki" <rjw@rjwysocki.net>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Linux PM <linux-pm@vger.kernel.org>,
        LKML <linux-kernel@vger.kernel.org>
References: <20220613120755.14306-1-peter.wang@mediatek.com>
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 <CAJZ5v0h8xNCV+1YwRA5wob6Vnvz8JFikv3pYMR_mUrXxzfc=tQ@mail.gmail.com>
From:   Peter Wang <peter.wang@mediatek.com>
Message-ID: <f6ebfd39-a27a-8b1c-6a61-f9a63236961d@mediatek.com>
Date:   Thu, 30 Jun 2022 23:19:15 +0800
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On 6/30/22 10:47 PM, Rafael J. Wysocki wrote:
> On Thu, Jun 30, 2022 at 4:26 PM Peter Wang <peter.wang@mediatek.com> wrote:
>>
>> On 6/30/22 12:01 AM, Rafael J. Wysocki wrote:
>>> [Add CCs to linix-pm, LKML and Greg]
>>>
>>> On Wednesday, June 29, 2022 5:32:00 PM CEST Rafael J. Wysocki wrote:
>>>> On Wed, Jun 29, 2022 at 4:47 PM Peter Wang <peter.wang@mediatek.com> wrote:
>>>>> On 6/29/22 9:22 PM, Rafael J. Wysocki wrote:
>>>>>> On Wed, Jun 29, 2022 at 5:02 AM Peter Wang <peter.wang@mediatek.com> wrote:
>>>>>>> On 6/28/22 11:54 PM, Rafael J. Wysocki wrote:
>>>>>>>> On Tue, Jun 28, 2022 at 3:53 AM Peter Wang <peter.wang@mediatek.com> wrote:
>>>>>>>>> On 6/28/22 3:00 AM, Rafael J. Wysocki wrote:
>>>>>>>>>> On Mon, Jun 13, 2022 at 2:08 PM <peter.wang@mediatek.com> wrote:
>>>>>>>>>>> From: Peter Wang <peter.wang@mediatek.com>
>>>>>>>>>>>
>>>>>>>>>>> With divice link of DL_FLAG_PM_RUNTIME, if consumer call pm_runtime_get_suppliers
>>>>>>>>>>> to prevent supplier enter suspend, pm_runtime_release_supplier should
>>>>>>>>>>> check supplier_preactivated before let supplier enter suspend.
>>>>>>>>>> Why?
>>>>>>>>> because supplier_preactivated is true means supplier cannot enter
>>>>>>>>> suspend, right?
>>>>>>>> No, it doesn't mean that.
>>>>>>> Hi Rafael,
>>>>>>>
>>>>>>> if supplier_preactivated is true, means someone call
>>>>>>> pm_runtime_get_suppliers and
>>>>>>> before pm_runtime_put_suppliers right? This section suppliers should not
>>>>>>> enter suspend.
>>>>>> No, this is not how this is expected to work.
>>>>>>
>>>>>> First off, the only caller of pm_runtime_get_suppliers() and
>>>>>> pm_runtime_put_suppliers() is __driver_probe_device().  Really nobody
>>>>>> else has any business that would require calling them.
>>>>> Hi Rafael,
>>>>>
>>>>> Yes, you are right!
>>>>> __driver_probe_device the only one use and just because
>>>>> __driver_probe_device use
>>>>> pm_runtime_get_suppliers cause problem.
>>>>>
>>>>>
>>>>>> Second, the role of pm_runtime_get_suppliers() is to "preactivate" the
>>>>>> suppliers before running probe for a consumer device and the role of
>>>>> the role of pm_runtime_get_suppliers() is to "preactivate" the suppliers,
>>>>> but suppliers may suspend immediately after preactivate right?
>>>>> Here is just this case. this is first racing point.
>>>>> Thread A: pm_runtime_get_suppliers                -> __driver_probe_device
>>>>> Thread B: pm_runtime_release_supplier
>>>>> Thread A: Run with supplier not preactivate      -> __driver_probe_device
>>>>>
>>>>>> pm_runtime_put_suppliers() is to do the cleanup in case the device is
>>>>>> left in suspend after probing.
>>>>>>
>>>>>> IOW, pm_runtime_get_suppliers() is to ensure that the suppliers will
>>>>>> be active until the probe callback takes over and the rest depends on
>>>>>> that callback.
>>>>> The problem of this racing will finally let consumer is active but
>>>>> supplier is suspended.
>>>> So it would be better to send a bug report regarding this.
>>>>
>>>>> The link relation is broken.
>>>>> I know you may curious how it happened? right?
>>>>> Honestly, I am not sure, but I think the second racing point
>>>>> is rpm_get_suppliers and pm_runtime_put_suppliers(release rpm_active).
>>>> I'm not sure what you mean by "the racing point".
>>>>
>>>> Yes, these functions can run concurrently.
>>>>
>>>>> So, I try to fix the first racing point and the problem is gone.
>>>>> It is full meet expect, and the pm runtime will work smoothly after
>>>>> __driver_probe_device done.
>>>> I'm almost sure that there is at least one scenario that would be
>>>> broken by this change.
>>> That said, the code in there may be a bit overdesigned.
>>>
>>> Does the patch below help?
>>>
>>> ---
>>>    drivers/base/power/runtime.c |   14 +-------------
>>>    1 file changed, 1 insertion(+), 13 deletions(-)
>>>
>>> Index: linux-pm/drivers/base/power/runtime.c
>>> ===================================================================
>>> --- linux-pm.orig/drivers/base/power/runtime.c
>>> +++ linux-pm/drivers/base/power/runtime.c
>>> @@ -1768,7 +1768,6 @@ void pm_runtime_get_suppliers(struct dev
>>>                if (link->flags & DL_FLAG_PM_RUNTIME) {
>>>                        link->supplier_preactivated = true;
>>>                        pm_runtime_get_sync(link->supplier);
>>> -                     refcount_inc(&link->rpm_active);
>>>                }
>>>
>>>        device_links_read_unlock(idx);
>>> @@ -1788,19 +1787,8 @@ void pm_runtime_put_suppliers(struct dev
>>>        list_for_each_entry_rcu(link, &dev->links.suppliers, c_node,
>>>                                device_links_read_lock_held())
>>>                if (link->supplier_preactivated) {
>>> -                     bool put;
>>> -
>>>                        link->supplier_preactivated = false;
>>> -
>>> -                     spin_lock_irq(&dev->power.lock);
>>> -
>>> -                     put = pm_runtime_status_suspended(dev) &&
>>> -                           refcount_dec_not_one(&link->rpm_active);
>>> -
>>> -                     spin_unlock_irq(&dev->power.lock);
>>> -
>>> -                     if (put)
>>> -                             pm_runtime_put(link->supplier);
>>> +                     pm_runtime_put(link->supplier);
>>>                }
>>>
>>>        device_links_read_unlock(idx);
>>
>> Hi Rafael,
>>
>> I think this patch solve the rpm_active racing problem.
>> But it still have problem that
>> pm_runtime_get_suppliers call pm_runtime_get_sync(link->supplier)
>> and supplier could suspend immediately by other thread who call
>> pm_runtime_release_supplier.
> No, it won't, because pm_runtime_release_supplier() won't drop the
> reference on the supplier taken by pm_runtime_get_suppliers(0 after
> the patch.

Hi Rafael,

I think pm_runtime_release_supplier will always decrese the reference
rpm_active count to 1 and check idle will let supplier enter suspend. Am 
I wrong?
Could you explain why this patch won't drop the reference?

Thanks

Peter