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

[Frank Rowand]

Hi Michael,

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.
> 
> 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.

Thanks for finding the problem, analyzing it, creating a unittest, and
creating a fix.

I agree with your analysis that there are issues with the implementation
of the test and fix.  I'll dig into this to see if I can provide some
useful improvements.

-Frank

> 
> Thanks,
>  Michael
> 
> 1. Note that this isn't a very good unit test: it will report a "pass"
>    even if it fails with the aforementioned errors, as these errors
>    aren't propogated.
> 
> Michael Auchter (3):
>   of: unittest: add test of overlay with devlinks
>   driver core: add device_links_barrier
>   of: dynamic: add device links barrier before detach
> 
>  drivers/base/core.c                     | 10 ++++++++++
>  drivers/of/dynamic.c                    |  3 +++
>  drivers/of/unittest-data/Makefile       |  1 +
>  drivers/of/unittest-data/overlay_16.dts | 26 +++++++++++++++++++++++++
>  drivers/of/unittest.c                   | 16 +++++++++++++++
>  include/linux/device.h                  |  1 +
>  6 files changed, 57 insertions(+)
>  create mode 100644 drivers/of/unittest-data/overlay_16.dts
>