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

Heikki Krogerus
2021-09-09T12:39:43Z

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 :-)

--
heikki