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Date:   Tue, 12 Jan 2021 19:24:24 +0100
From:   "Rafael J. Wysocki" <rafael@...nel.org>
To:     Stephan Gerhold <stephan@...hold.net>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Peter Chen <Peter.Chen@....com>,
        Kishon Vijay Abraham I <kishon@...com>,
        "open list:ULTRA-WIDEBAND (UWB) SUBSYSTEM:" 
        <linux-usb@...r.kernel.org>, Thierry Reding <treding@...dia.com>
Subject: Re: Infinite recursion in device_reorder_to_tail() due to circular
 device links

On Tue, Jan 12, 2021 at 3:32 PM Rafael J. Wysocki <rafael@...nel.org> wrote:
>
> On Mon, Jan 11, 2021 at 7:46 PM Stephan Gerhold <stephan@...hold.net> 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.

I've got it the other way around, so it should have been:

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

Sorry.

That said, device_is_dependend() doesn't really check for "family
connections", so to speak, which it should do in principle.

That is, it should return "true" if "target" or any direct ancestor of
it is any of the devices that depend on "dev", not just when "target"
itself is any of those devices.

Let me cut a patch for that.

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