linux-kernel - Re: [PATCH] software node: balance refcount for managed sw nodes

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Date:   Tue, 20 Jul 2021 12:20:40 +0300
From:   Laurentiu Tudor <laurentiu.tudor@....com>
To:     Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
Cc:     Jon Nettleton <jon@...id-run.com>,
        Heikki Krogerus <heikki.krogerus@...ux.intel.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "Rafael J . Wysocki" <rafael@...nel.org>,
        ACPI Devel Maling List <linux-acpi@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] software node: balance refcount for managed sw nodes



On 7/19/2021 3:22 PM, Andy Shevchenko wrote:
> On Mon, Jul 19, 2021 at 03:00:17PM +0300, Laurentiu Tudor wrote:
>> On 7/16/2021 8:21 PM, Jon Nettleton wrote:
>>> On Fri, Jul 16, 2021 at 2:17 PM Andy Shevchenko
>>> <andriy.shevchenko@...ux.intel.com> wrote:
>>>>
>>>> On Fri, Jul 16, 2021 at 01:16:02PM +0300, laurentiu.tudor@....com wrote:
>>>>> From: Laurentiu Tudor <laurentiu.tudor@....com>
>>>>>
>>>>> software_node_notify(), on KOBJ_REMOVE drops the refcount twice on managed
>>>>> software nodes, thus leading to underflow errors. Balance the refcount by
>>>>> bumping it in the device_create_managed_software_node() function.
>>>>>
>>>>> The error [1] was encountered after adding a .shutdown() op to our
>>>>> fsl-mc-bus driver.
>>>>
>>>> Looking into the history of adding ->shutdown() to dwc3 driver (it got reverted
>>>> later on), I can tell that probably something is wrong in the ->shutdown()
>>>> method itself.
>>>
>>> Isn't the other alternative to just remove the second kobject_put from
>>> KOBJ_REMOVE ?
>>>
>>
>> Or maybe on top of Heikki's suggestion, replace the calls to
>> sysfs_create_link() from KOBJ_ADD with sysfs_create_link_nowarn()?
> 
> _noearn will hide the problem. It was there, it was removed from there.
> Perhaps we have to understand the root cause better (some specific flow?).
> 
> Any insight from you on the flow when the issue appears? I.o.w. what happened
> on the big picture that we got into the warning you see?
> 

I encountered the initial issue when trying to shut down a system booted
with ACPI but only after adding a .shutdown() callback to our bus driver
so that the devices are properly taken down. The problem was that
software_node_notify(), on KOBJ_REMOVE was dropping the reference count
twice leading to an underflow error. My initial proposal was to just
bump the refcount in device_create_managed_software_node(). The device
properties that triggered the problem are created here [1].

Heikko suggested that instead of manually incrementing the refcount to
use software_node_notify(KOBJ_ADD). This triggered the second issue, a
duplicated sysfs entry warning originating in the usb subsystem:
device_create_managed_software_node() ends up being called twice, once
here [2] and secondly, the place I previous mentioned [1].

[1]
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/drivers/acpi/arm64/iort.c#n952
[2]
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/drivers/usb/dwc3/host.c#n111

---
Best Regards, Laurentiu