Messages in this thread Patch in this message | | | Subject | Re: [PATCH v3] driver core: Break infinite loop when deferred probe can't be satisfied | From | Grant Likely <> | Date | Thu, 26 Mar 2020 18:06:37 +0000 |
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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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