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

Andy Shevchenko
2021-07-20T10:31:31Z

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?

> [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

[3]: https://lore.kernel.org/linux-usb/CAHp75Vd-5U5zgtDfM5C3Jsx51HVYB+rNcHYC2XP=G7dOd=cdTg@mail.gmail.com/
[4]: https://lore.kernel.org/linux-usb/c3c75895-313a-5be7-6421-b32bac741a88@arm.com/T/#u
[5]: https://elixir.bootlin.com/linux/latest/source/drivers/base/core.c#L2216
[6]: https://elixir.bootlin.com/linux/latest/source/drivers/base/core.c#L4447

--
With Best Regards,
Andy Shevchenko