Re: [PATCH v3] driver core: Break infinite loop when deferred probe can't be satisfied

[Grant Likely <grant.likely@arm.com>]

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.

It can be solved by refactoring the driver probe routine. If a resource 
is required to be present, then check that it is available early; before 
registering child devices.

The proposed solution to modify driver core is fragile and susceptible 
to side effects from other probe paths. I don't think it is the right 
approach.

g.