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Subject: Re: [PATCH v1] PM-runtime: Check supplier_preactivated before release
 supplier
To:     "Rafael J. Wysocki" <rjw@rjwysocki.net>
CC:     "Rafael J. Wysocki" <rafael@kernel.org>,
        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>
 <b55d5691-0b2d-56bb-26ff-dcac56770611@mediatek.com>
 <CAJZ5v0gTpv2gt_Gm9rUd+8Jmp4=ij2=J20o7qO0sC-hm=w3=_A@mail.gmail.com>
 <12028598.O9o76ZdvQC@kreacher>
From:   Peter Wang <peter.wang@mediatek.com>
Message-ID: <90b5f619-2dd6-817b-fe2d-f895be0b5b98@mediatek.com>
Date:   Thu, 30 Jun 2022 22:26:04 +0800
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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.

Thanks.
Peter


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