Messages in this thread Patch in this message |  | | Date | Fri, 27 Mar 2020 09:03:23 +0100 | From | Greg KH <> | Subject | Re: [PATCH v3] driver core: Break infinite loop when deferred probe can't be satisfied |
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On Thu, Mar 26, 2020 at 06:06:37PM +0000, Grant Likely wrote: > > > 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.
If all child devices really are cleaned up completly, why would this be a problem? What is set internally in the driver core that would get tripped up by this?
> 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.
Ok, this is a bug, if that is the case, in the driver core as it should not matter how many devices were added/removed/whatever while a driver is in it's probe function.
But, I don't see how this patch solves that problem, another probe call should never be made for the same bus while in this probe function. If we do: device1->probe() device1 creates device2 and registers it device2->probe is called device2->probe returns 0 device1 has problems, unregisters device2 device2->remove is called device1 deletes device2 device1 returns -EPROBE_DEFER
So then where's the problem? Did device2 somehow not really get properly cleaned up?
confused,
greg k-h
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