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Date:   Tue, 12 Jan 2021 15:32:04 +0100
From:   "Rafael J. Wysocki" <>
To:     Stephan Gerhold <>,
        Greg Kroah-Hartman <>
Cc:     "Rafael J. Wysocki" <>,
        Linux Kernel Mailing List <>,
        Peter Chen <>,
        Kishon Vijay Abraham I <>,
        "open list:ULTRA-WIDEBAND (UWB) SUBSYSTEM:" 
        <>, Thierry Reding <>
Subject: Re: Infinite recursion in device_reorder_to_tail() due to circular
 device links

On Mon, Jan 11, 2021 at 7:46 PM Stephan Gerhold <> wrote:
> Hi,
> since 5.11-rc1 I get kernel crashes with infinite recursion in
> device_reorder_to_tail() in some situations... It's a bit complicated to
> explain so I want to apologize in advance for the long mail. :)
>   Kernel panic - not syncing: kernel stack overflow
>   CPU: 1 PID: 33 Comm: kworker/1:1 Not tainted 5.11.0-rc3 #1
>   Hardware name: Qualcomm Technologies, Inc. APQ 8016 SBC (DT)
>   Call trace:
>    ...
>    device_reorder_to_tail+0x4c/0xf0
>    device_reorder_to_tail+0x98/0xf0
>    device_reorder_to_tail+0x60/0xf0
>    device_reorder_to_tail+0x60/0xf0
>    device_reorder_to_tail+0x60/0xf0
>    ...
> The crash happens only in 5.11 with commit 5b6164d3465f ("driver core:
> Reorder devices on successful probe"). It stops happening when I revert
> this commit.

Thanks for the report!

Greg, please revert commit 5b6164d3465f, it clearly is not an
improvement, at least at this point.

> But I don't think this commit is the actual problem...

Well, it may not be the root cause, but it is a change in behavior
that exposes the breakage and this is not the only problem introduced
by it.

> It's easy to reproduce on any device based on the Qualcomm MSM8916 SoC
> by adding a random regulator to the USB node, e.g.:
> diff --git a/arch/arm64/boot/dts/qcom/apq8016-sbc.dtsi b/arch/arm64/boot/dts/qcom/apq8016-sbc.dtsi
> index 3a9538e1ec97..9f43fce9e6e3 100644
> --- a/arch/arm64/boot/dts/qcom/apq8016-sbc.dtsi
> +++ b/arch/arm64/boot/dts/qcom/apq8016-sbc.dtsi
> @@ -372,6 +372,7 @@ codec {
>  &usb {
>         status = "okay";
> +       vbus-supply = <&pm8916_l5>;
>         extcon = <&usb_id>, <&usb_id>;
>         pinctrl-names = "default", "device";
> I searched for problems in the regulator core but the problem actually
> has nothing to do with regulators: The additional regulator supply just
> delays probing of the USB driver long enough to trigger the issue.
> Adding some debug output to device_reorder_to_tail() reveals that it
> keeps recursing over the same 4 devices:
>   msm_hsusb 78d9000.usb: device_reorder_to_tail()
>   ci_hdrc ci_hdrc.0: device_reorder_to_tail()
>   qcom_usb_hs_phy ci_hdrc.0.ulpi: device_reorder_to_tail()
>   phy phy-ci_hdrc.0.ulpi.0: device_reorder_to_tail()
>   msm_hsusb 78d9000.usb: device_reorder_to_tail()
>   ...
> The device hierarchy of these is (children devices):
>   78d9000.usb -> ci_hdrc.0 -> ci_hdrc.0.ulpi -> phy-ci_hdrc.0.ulpi.0
> ci_hdrc.0 calls phy_get(dev->parent, "usb-phy"). In phy_get(),
> the phy-core then attempts to add the following device link:
>   phy-ci_hdrc.0.ulpi.0 -> 78d9000.usb
> The device link creation in phy-core is optional (see commit 1d7cb11e1090
> ("phy: core: Fix phy_get() to not return error on link creation failure"))
> because this device link is circular in case of ULPI PHYs (like here).
> And indeed, creating this device link usually fails (as it should).
> However, adding the "vbus-supply" changes probe order in some way that
> this circular device link ends up being created:
>   /sys/class/devlink/phy-ci_hdrc.0.ulpi.0--78d9000.usb/ exists only when
> I add the "vbus-supply" as in the diff above.
> Apparently, there is a special situation where device_is_dependent()
> does not work properly, and therefore the driver core allows creating
> the circular device link.
> To show the problem, I enabled some debug messages and added the
> following log message:
> diff --git a/drivers/base/core.c b/drivers/base/core.c
> index 25e08e5f40bd..ff1344eabb31 100644
> --- a/drivers/base/core.c
> +++ b/drivers/base/core.c
> @@ -3089,9 +3089,11 @@ int device_add(struct device *dev)
>         }
>         bus_probe_device(dev);
> -       if (parent)
> +       if (parent) {
> +               dev_info(dev, "add to parent %s\n", dev_name(parent));
>                 klist_add_tail(&dev->p->knode_parent,
>                                &parent->p->klist_children);
> +       }
>         if (dev->class) {
>                 mutex_lock(&dev->class->p->mutex);
> Running this with "vbus-supply" (where it crashes) produces:
>      bus: 'platform': probing driver msm_hsusb with device 78d9000.usb
>        <some probe deferrals while waiting for the regulator>
>      bus: 'platform': probing driver msm_hsusb with device 78d9000.usb
>      bus: 'platform': probing driver ci_hdrc with device ci_hdrc.0
>      bus: 'ulpi': probing driver qcom_usb_hs_phy with device ci_hdrc.0.ulpi
>      phy phy-ci_hdrc.0.ulpi.0: add to parent ci_hdrc.0.ulpi
>      qcom_usb_hs_phy ci_hdrc.0.ulpi: add to parent ci_hdrc.0
>  (1) msm_hsusb 78d9000.usb: Linked as a consumer to phy-ci_hdrc.0.ulpi.0
>  (2) ci_hdrc ci_hdrc.0: add to parent 78d9000.usb
>      Kernel panic - not syncing: kernel stack overflow
>       ...
> Note how ci_hdrc is added to the children list of 78d9000.usb (2) after
> the device link was already created in (1). This is why device_is_dependent()
> does not realize the devices will eventually be dependent on each other.

Well, it cannot know beforehand that the consumer is going to be
registered as a child of the supplier.

> Without "vbus-supply" (2) happens before (1) because then the PHY driver
> requests probe deferral a few times while waiting for regulators.
> I'm not really sure what to do here, any suggestions how to fix this properly?

Clearly, device_add() needs to check if there is a device link between
dev and dev->parent, but it is not so clear what to do if such a link
is present.

I would make it return an error in that case.

> (In case it helps, I have attached an example kernel module (circdl.c)
>  that allows reproducing the exact same crash.)

IMV the bug is in the code that attempts to create a device link
before registering the child and it clearly shows that creating device
links from unregistered consumers is not really a good idea in

In principle, a device link creation flag to instruct
device_link_add() to fail if the consumer is not registered can be
added and it can be used by the code in question if that helps.

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