Re: Re: [RFC PATCH 0/3] Fix errors on DT overlay removal with devlinks

[Michael Auchter]

On Thu, Oct 29, 2020 at 03:54:21PM -0500, Frank Rowand wrote:
> On 10/28/20 11:25 AM, Michael Auchter wrote:
> > Hey Saravana,
> > 
> > Thanks for taking the time to look into this!
> > 
> > On Mon, Oct 26, 2020 at 12:10:33PM -0700, Saravana Kannan wrote:
> >> On Wed, Oct 21, 2020 at 2:02 PM Frank Rowand <frowand.list@xxxxxxxxx> wrote:
> >>>
> >>> Hi Saravana,
> >>>
> >>> Michael found an issue related to the removal of a devicetree node
> >>> which involves devlinks:
> >>>
> >>> On 10/14/20 2:36 PM, Michael Auchter wrote:
> >>>> After updating to v5.9, I've started seeing errors in the kernel log
> >>>> when using device tree overlays. Specifically, the problem seems to
> >>>> happen when removing a device tree overlay that contains two devices
> >>>> with some dependency between them (e.g., a device that provides a clock
> >>>> and a device that consumes that clock). Removing such an overlay results
> >>>> in:
> >>>>
> >>>>   OF: ERROR: memory leak, expected refcount 1 instead of 2, of_node_get()/of_node_put() unbalanced - destroy
> >>>>   OF: ERROR: memory leak, expected refcount 1 instead of 2, of_node_get()/of_node_put() unbalanced - destroy
> >>>>
> >>>> followed by hitting some REFCOUNT_WARNs in refcount.c
> >>>>
> >>>> In the first patch, I've included a unittest that can be used to
> >>>> reproduce this when built with CONFIG_OF_UNITTEST [1].
> >>>>
> >>>> I believe the issue is caused by the cleanup performed when releasing
> >>>> the devlink device that's created to represent the dependency between
> >>>> devices. The devlink device has references to the consumer and supplier
> >>>> devices, which it drops in device_link_free; the devlink device's
> >>>> release callback calls device_link_free via call_srcu.
> >>>>
> >>>> When the overlay is being removed, all devices are removed, and
> >>>> eventually the release callback for the devlink device run, and
> >>>> schedules cleanup using call_srcu. Before device_link_free can and call
> >>>> put_device on the consumer/supplier, the rest of the overlay removal
> >>>> process runs, resulting in the error traces above.
> >>>
> >>> When a devicetree node in an overlay is removed, the remove code expects
> >>> all previous users of the related device to have done the appropriate put
> >>> of the device and to have no later references.
> >>>
> >>> As Michael described above, the devlink release callback defers the
> >>> put_device().  The cleanup via srcu was implemented in commit
> >>> 843e600b8a2b01463c4d873a90b2c2ea8033f1f6 "driver core: Fix sleeping
> >>> in invalid context during device link deletion" to solve yet another
> >>> issue.
> >>>
> >>>
> >>>>
> >>>> Patches 2 and 3 are an attempt at fixing this: call srcu_barrier to wait
> >>>> for any pending device_link_free's to execute before continuing on with
> >>>> the removal process.
> >>>>
> >>>> These patches resolve the issue, but probably not in the best way. In
> >>>> particular, it seems strange to need to leak details of devlinks into
> >>>> the device tree overlay code. So, I'd be curious to get some feedback or
> >>>> hear any other ideas for how to resolve this issue.
> >>>
> >>> I agree with Michael that adding an indirect call of srcu_barrier(&device_links_srcu)
> >>> into the devicetree overlay code is not an appropriate solution.
> >>
> >> I kind of see your point too. I wonder if the srcu_barrier() should
> >> happen inside like so:
> >> device_del() -> device_links_purge()->srcu_barrier()
> >>
> >> I don't know what contention the use of SRCUs in device links was
> >> trying to avoid, but I think the srcu_barrier() call path I suggested
> >> above shouldn't be a problem. If that fixes the issue, the best way to
> >> know if it's an issue is to send out a patch and see if Rafael has any
> >> problem with it :)
> > 
> > I was able to test this by adding the srcu_barrier() at the end of
> > device_links_purge(), and that does seem to have fixed the issue.
> > 
> >>> Is there some other way to fix the problem that 843e600b8a2b solves without
> >>> deferring the put_device() done by the devlink release callback?
> >>
> >> Ok I finally got some time to look into this closely.
> >>
> >> Even if you revert 843e600b8a2b, you'll see that device_link_free()
> >> (which drops the reference to the consumer and supplier devices) was
> >> scheduled to run when the SRCU clean up occurs. So I think this issue
> >> was present even before 843e600b8a2b, but commit 843e600b8a2b just
> >> made it more likely to hit this scenario because it introduces some
> >> delay in dropping the ref count of the supplier and consumer by going
> >> through the device link device's release path. So, I think this issue
> >> isn't related to 843e600b8a2b.
> >>
> >> As to why 843e600b8a2b had to be written to call call_srcu() from the
> >> device link device's release path, it's a mess of dependencies/delays:
> >> 1. The device link device is part of the struct device_link. So we
> >> can't free device_link before the device_link.link_dev refcount goes
> >> to 0.
> >> 2. But I can't assume device_link.link_dev's refcount will go to 0 as
> >> soon as I call put_device() on it because of
> >> CONFIG_DEBUG_KOBJECT_RELEASE which frees up the kobject after a random
> >> delay.
> >> 3. The use of SRCU also means I can't free device_link until the SRCU
> >> is cleaned up.
> >>
> >> Because of (1), (2) and (3), when the device_link_del() (or any of the
> >> other device link deletion APIs are called) I first have to do a
> >> put_device(device_link.link_dev) to make sure the device memory is no
> >> longer referenced, then trigger an SRCU clean up and then in the
> >> scheduled SRCU cleanup I can free struct device_link. And obviously,
> >> until struct device_link is ready to be freed up, I can't drop the
> >> reference to the supplier and consumer devices (as that old copy of
> >> device_link could be used by some code to refer to the supplier and
> >> consumer devices).
> >>
> >> Hope that helps explain the SRCU and device link device release dependencies.
> >>
> >> Also, even if this patch series is applied as is, I wonder if the
> >> current overlay code has a bug related to CONFIG_DEBUG_KOBJECT_RELEASE
> >> delaying the actual freeing of the device. Something to look into?
> > 
> > I also tried enabling CONFIG_DEBUG_KOBJECT_RELEASE... with or without
> > the addition of srcu_barrier() to device_links_purge(), I can't boot
> > successfully when CONFIG_OF_UNITTEST=y &&
> > CONFIG_DEBUG_KOBJECT_RELEASE=y: there are a ton of errors that result
> > from this combo.
> 
> I'll add looking checking out booting with CONFIG_DEBUG_KOBJECT_RELEASE
> enabled with CONFIG_OF_UNITTEST enabled to my todo list.
> 
> > 
> > Disabling the unittests and booting with CONFIG_DEBUG_KOBJECT_RELEASE=y,
> > I _do_ still see the errors mentioned in my original message when
> > removing an overlay. So yeah, it does seem like there are some latent
> 
> Just to make sure I understand clearly, you are still seeing the
> messages:
> 
>    OF: ERROR: memory leak, expected refcount 1 instead of 2, of_node_get()/of_node_put() unbalanced - destroy
>    OF: ERROR: memory leak, expected refcount 1 instead of 2, of_node_get()/of_node_put() unbalanced - destroy
> 
> when the overlay is removed?  And if I apply patch 1/3 (the new unittest)
> and also add srcu_barrier() at the end of device_links_purge()
> then I will see these messages?

Yes, that's correct: with CONFIG_DEBUG_KOBJECT_RELEASE=y + the new
unittest, I see those errors when booting, regardless of whether the
srcu_barrier is present in device_links_purge().

> Can you add a reply to this email with the patch to add srcu_barrier() at
> the end of device_links_purge() so that I am testing the same thing that
> you are testing?  (If it makes life easier for you, you can just cut
> and paste the patch into the reply, even if it results in a white
> space damaged patch -- I'm assuming a one-line patch.)

Sure thing: