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

Laurentiu Tudor
2021-09-10T13:01:00Z

Hi Heikki,

On 9/10/2021 3:38 PM, Heikki Krogerus wrote:
> On Fri, Sep 10, 2021 at 03:05:16PM +0300, Laurentiu Tudor wrote:
>>
>>
>> On 9/9/2021 5:01 PM, Laurentiu Tudor wrote:
>>>
>>>
>>> On 9/9/2021 3:16 PM, Heikki Krogerus wrote:
>>>> On Thu, Sep 09, 2021 at 03:13:47PM +0300, Heikki Krogerus wrote:
>>>>> On Tue, Sep 07, 2021 at 06:59:18PM +0300, Laurentiu Tudor wrote:
>>>>>>
>>>>>>
>>>>>> On 7/26/2021 10:59 AM, Laurentiu Tudor wrote:
>>>>>>>
>>>>>>>
>>>>>>> On 7/20/2021 1:27 PM, Andy Shevchenko wrote:
>>>>>>>> On Tue, Jul 20, 2021 at 12:22 PM Laurentiu Tudor
>>>>>>>> <laurentiu.tudor@nxp.com> wrote:
>>>>>>>>> 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@linux.intel.com> wrote:
>>>>>>>>>>>>>
>>>>>>>>>>>>> On Fri, Jul 16, 2021 at 01:16:02PM +0300, laurentiu.tudor@nxp.com wrote:
>>>>>>>>>>>>>> From: Laurentiu Tudor <laurentiu.tudor@nxp.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].
>>>>>>>>
>>>>>>>> This [3] is what I have reported against DWC3 when ->shutdown() has
>>>>>>>> been added there. And here [4] is another thread about the issue with
>>>>>>>> that callback. The ->release() callback is called at put_device() [5]
>>>>>>>> and ->shutdown() is called before that [6]. That said, can you inspect
>>>>>>>> your ->shutdown() implementation once more time and perhaps see if
>>>>>>>> there is anything that can be amended?
>>>>>>>>
>>>>>>>
>>>>>>> Will do, thanks for the pointers. It could be that we mess something out
>>>>>>> in how we use the driver model.
>>>>>>>
>>>>>>
>>>>>> Quick (and late, sorry) update from my side. I've spent time on
>>>>>> debugging our bus, did found some issues but, at least for now, none are
>>>>>> related to sw node.
>>>>>> In the mean time, I noticed in the swnode code that
>>>>>> device_add_software_node() calls software_node_notify(KOBJ_ADD) while
>>>>>> device_create_managed_software_node() doesn't. Updating [1] the later
>>>>>> with the call to software_node_notify(KOBJ_ADD) does seem to fix the
>>>>>> issue I'm seeing.
>>>>>>
>>>>>> Could this be a problem? Any comments appreciated.
>>>>>>
>>>>>> One more thing perhaps worth mentioning is that, at least for now, there
>>>>>> are few uses for this device_create_managed_software_node() api,
>>>>>> mentioning here a couple of them:
>>>>>> - arm64 iort code - this seems to be triggering the issue i'm getting
>>>>>> - dwc3 usb - Andy reported similar issues here, maybe the issue is common?
>>>>>>
>>>>>> [1]
>>>>>> @@ -1113,6 +1125,15 @@ int device_create_managed_software_node(struct
>>>>>> device *dev,
>>>>>> to_swnode(fwnode)->managed = true;
>>>>>> set_secondary_fwnode(dev, fwnode);
>>>>>>
>>>>>> + /*
>>>>>> + * If the device has been fully registered by the time this
>>>>>> function is
>>>>>> + * called, software_node_notify() must be called separately so
>>>>>> that the
>>>>>> + * symlinks get created and the reference count of the node is
>>>>>> kept in
>>>>>> + * balance.
>>>>>> + */
>>>>>> + if (device_is_registered(dev))
>>>>>> + software_node_notify(dev, KOBJ_ADD);
>>>>>> +
>>>>>> return 0;
>>>>>> }
>>>>>
>>>>> That should be fixed indeed. Please send that after -rc1 is out.
>>>>
>>>> I mean, resend :-)
>>>>
>>>
>>> Right, actually I just noticed that this is the fix you suggested last
>>> time we discussed. :-) I forgot about it, sorry.
>>> There's still the WARN_ON() [1] triggered by the usb subsys, apparently
>>> happening only (!) in ACPI boot scenario, so +Lorenzo.
>>> I'll delay the sending a bit to try to understand what's going on.
>>
>> I've spent some time looking into this and it turns out that in the
>> ACPI case, device_create_managed_software_node() ends up being called
>> twice, first here [1] and after that, in the IORT code here [2]. With
>> the proposed patch this causes software_node_notify(KOBJ_ADD) being
>> called twice thus triggering the dup sysfs entry warning.
>> Any comments / ideas welcomed.
>>
>> [1] https://elixir.bootlin.com/linux/latest/source/drivers/usb/dwc3/host.c#L111
>
> I think the problem here is that the secondary fwnode get's replaced
> because the primary fwnode is shared. Can you test it with this, just
> to see if the problem goes away:
>
> diff --git a/drivers/usb/dwc3/host.c b/drivers/usb/dwc3/host.c
> index f29a264635aa1..e4b40f8b8f242 100644
> --- a/drivers/usb/dwc3/host.c
> +++ b/drivers/usb/dwc3/host.c
> @@ -76,7 +76,6 @@ int dwc3_host_init(struct dwc3 *dwc)
> }
>
> xhci->dev.parent = dwc->dev;
> - ACPI_COMPANION_SET(&xhci->dev, ACPI_COMPANION(dwc->dev));
>
> dwc->xhci = xhci;

Thanks for looking into this! Yes, this does make the issue go away.

---
Best Regards, Laurentiu