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    SubjectRe: [PATCH v3] driver core: Break infinite loop when deferred probe can't be satisfied
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    On 26/03/2020 16:39, Greg KH wrote:
    > On Thu, Mar 26, 2020 at 06:31:10PM +0200, Andy Shevchenko wrote:
    >> On Thu, Mar 26, 2020 at 03:01:22PM +0000, Grant Likely wrote:
    >>> On 25/03/2020 12:51, Andy Shevchenko wrote:
    >>>> On Tue, Mar 24, 2020 at 08:29:01PM -0700, Saravana Kannan wrote:
    >>>>> On Tue, Mar 24, 2020 at 5:38 AM Andy Shevchenko <andriy.shevchenko@linux.intel.com> wrote:
    >>>>>> Consider the following scenario.
    >>>>>>
    >>>>>> The main driver of USB OTG controller (dwc3-pci), which has the following
    >>>>>> functional dependencies on certain platform:
    >>>>>> - ULPI (tusb1210)
    >>>>>> - extcon (tested with extcon-intel-mrfld)
    >>>>>>
    >>>>>> Note, that first driver, tusb1210, is available at the moment of
    >>>>>> dwc3-pci probing, while extcon-intel-mrfld is built as a module and
    >>>>>> won't appear till user space does something about it.
    >>>>>>
    >>>>>> This is depicted by kernel configuration excerpt:
    >>>>>>
    >>>>>> CONFIG_PHY_TUSB1210=y
    >>>>>> CONFIG_USB_DWC3=y
    >>>>>> CONFIG_USB_DWC3_ULPI=y
    >>>>>> CONFIG_USB_DWC3_DUAL_ROLE=y
    >>>>>> CONFIG_USB_DWC3_PCI=y
    >>>>>> CONFIG_EXTCON_INTEL_MRFLD=m
    >>>>>>
    >>>>>> In the Buildroot environment the modules are probed by alphabetical ordering
    >>>>>> of their modaliases. The latter comes to the case when USB OTG driver will be
    >>>>>> probed first followed by extcon one.
    >>>>>>
    >>>>>> So, if the platform anticipates extcon device to be appeared, in the above case
    >>>>>> we will get deferred probe of USB OTG, because of ordering.
    >>>>>>
    >>>>>> Since current implementation, done by the commit 58b116bce136 ("drivercore:
    >>>>>> deferral race condition fix") counts the amount of triggered deferred probe,
    >>>>>> we never advance the situation -- the change makes it to be an infinite loop.
    >>>>>
    >>>>> Hi Andy,
    >>>>>
    >>>>> I'm trying to understand this sequence of steps. Sorry if the questions
    >>>>> are stupid -- I'm not very familiar with USB/PCI stuff.
    >>>>
    >>>> Thank you for looking into this. My answer below.
    >>>>
    >>>> As a first thing I would like to tell that there is another example of bad
    >>>> behaviour of deferred probe with no relation to USB. The proposed change also
    >>>> fixes that one (however, less possible to find in real life).
    >>>>
    >>>>>> ---8<---8<---
    >>>>>>
    >>>>>> [ 22.187127] driver_deferred_probe_trigger <<< 1
    >>>>>>
    >>>>>> ...here is the late initcall triggers deferred probe...
    >>>>>>
    >>>>>> [ 22.191725] platform dwc3.0.auto: deferred_probe_work_func in deferred list
    >>>>>>
    >>>>>> ...dwc3.0.auto is the only device in the deferred list...
    >>>>>
    >>>>> Ok, dwc3.0.auto is the only unprobed device at this point?
    >>>>
    >>>> Correct.
    >>>>
    >>>>>> [ 22.198727] platform dwc3.0.auto: deferred_probe_work_func 1 <<< counter 1
    >>>>>>
    >>>>>> ...the counter before mutex is unlocked is kept the same...
    >>>>>>
    >>>>>> [ 22.205663] platform dwc3.0.auto: Retrying from deferred list
    >>>>>>
    >>>>>> ...mutes has been unlocked, we try to re-probe the driver...
    >>>>>>
    >>>>>> [ 22.211487] bus: 'platform': driver_probe_device: matched device dwc3.0.auto with driver dwc3
    >>>>>> [ 22.220060] bus: 'platform': really_probe: probing driver dwc3 with device dwc3.0.auto
    >>>>>> [ 22.238735] bus: 'ulpi': driver_probe_device: matched device dwc3.0.auto.ulpi with driver tusb1210
    >>>>>> [ 22.247743] bus: 'ulpi': really_probe: probing driver tusb1210 with device dwc3.0.auto.ulpi
    >>>>>> [ 22.256292] driver: 'tusb1210': driver_bound: bound to device 'dwc3.0.auto.ulpi'
    >>>>>> [ 22.263723] driver_deferred_probe_trigger <<< 2
    >>>>>>
    >>>>>> ...the dwc3.0.auto probes ULPI, we got successful bound and bumped counter...
    >>>>>>
    >>>>>> [ 22.268304] bus: 'ulpi': really_probe: bound device dwc3.0.auto.ulpi to driver tusb1210
    >>>>>
    >>>>> So where did this dwc3.0.auto.ulpi come from?
    >>>>
    >>>>> Looks like the device is created by dwc3_probe() through this call flow:
    >>>>> dwc3_probe() -> dwc3_core_init() -> dwc3_core_ulpi_init() ->
    >>>>> dwc3_ulpi_init() -> ulpi_register_interface() -> ulpi_register()
    >>>>
    >>>> Correct.
    >>>>
    >>>>>> [ 22.276697] platform dwc3.0.auto: Driver dwc3 requests probe deferral
    >>>>>
    >>>>> Can you please point me to which code patch actually caused the probe
    >>>>> deferral?
    >>>>
    >>>> Sure, it's in drd.c.
    >>>>
    >>>> if (device_property_read_string(dev, "linux,extcon-name", &name) == 0) {
    >>>> edev = extcon_get_extcon_dev(name);
    >>>> if (!edev)
    >>>> return ERR_PTR(-EPROBE_DEFER);
    >>>> return edev;
    >>>> }
    >>>>
    >>>>>> ...but extcon driver is still missing...
    >>>>>>
    >>>>>> [ 22.283174] platform dwc3.0.auto: Added to deferred list
    >>>>>> [ 22.288513] platform dwc3.0.auto: driver_deferred_probe_add_trigger local counter: 1 new counter 2
    >>>>>
    >>>>> I'm not fully aware of all the USB implications, but if extcon is
    >>>>> needed, why can't that check be done before we add and probe the ulpi
    >>>>> device? That'll avoid this whole "fake" probing and avoid the counter
    >>>>> increase. And avoid the need for this patch that's touching the code
    >>>>> code that's already a bit delicate.
    >>>>
    >>>>> Also, with my limited experience with all the possible drivers in the
    >>>>> kernel, it's weird that the ulpi device is added and probed before we
    >>>>> make sure the parent device (dwc3.0.auto) can actually probe
    >>>>> successfully.
    >>>>
    >>>> As I said above the deferred probe trigger has flaw on its own.
    >>>> Even if we fix for USB case, there is (and probably will be) others.
    >>>
    >>> Right here is the driver design bug. A driver's probe() hook should *not*
    >>> return -EPROBE_DEFER after already creating child devices which may have
    >>> already been probed.
    >>
    >> Any documentation statement for this requirement?
    >
    > There shouldn't be. If you return ANY error from a probe function, your
    > driver is essencially "dead" when it comes to that device, and it had
    > better have cleaned up after itself. >
    > That includes defering probe, that's not "special" here at all.

    What is special in this case is that if a .probe() hook had registered a
    child device, then removed that child device (so it did clean up after
    itself) and then return -EPROBE_DEFER, then we end up in an endless
    probe loop.

    But this is unusual behaviour. Normally a .probe() hook checks all
    required resources are available before registering any child devices.
    This driver doesn't do that. Arguably this is indeed an additional
    requirement beyond "clean up after yourself". I cannot find anyplace
    where it is documented. In fact, I cannot find any documentation on
    EPROBE_DEFER in the Documentation/ tree. How about the below?

    >> By the way, I may imagine other mechanisms that probe the driver on other CPU
    >> at the same time (let's consider parallel modprobes). The current code has a
    >> flaw with that.
    >
    > That can't happen, the driver core prevents that.

    Greg's right, that can't happen. At worst a driver will get an
    additional defer event; but it all still works.

    g.

    ---
    diff --git a/Documentation/driver-api/driver-model/driver.rst
    b/Documentation/driver-api/driver-model/driver.rst
    index baa6a85c8287..46adede13aba 100644
    --- a/Documentation/driver-api/driver-model/driver.rst
    +++ b/Documentation/driver-api/driver-model/driver.rst
    @@ -167,7 +167,17 @@ the driver to that device.

    A driver's probe() may return a negative errno value to indicate that
    the driver did not bind to this device, in which case it should have
    -released all resources it allocated::
    +released all resources it allocated. Optionally, probe() may return
    +-EPROBE_DEFER if the driver depends on resources that are not yet
    +available (e.g., supplied by a driver that hasn't initialized yet).
    +The driver core will put the device onto the deferred probe list and
    +will try to call it again later. Important: -EPROBE_DEFER must not be
    +returned if probe() has already created child devices, even if those
    +child devices have were removed again in a cleanup path. If -EPROBE_DEFER
    +is returned after a child device has been registered, it may result in an
    +infinite loop of .probe() calls to the same driver.
    +
    +::

    void (*sync_state)(struct device *dev);
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