Messages in this thread | | | From | Saravana Kannan <> | Date | Thu, 26 Mar 2020 12:55:34 -0700 | 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 4:54 AM Andy Shevchenko <andriy.shevchenko@linux.intel.com> wrote: > > On Wed, Mar 25, 2020 at 03:08:29PM -0700, Saravana Kannan wrote: > > On Wed, Mar 25, 2020 at 5:51 AM Andy Shevchenko > > <andriy.shevchenko@linux.intel.com> 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). > > > > Unless I see what the other issue is, I can't speak for the unknown. > > Okay, let's talk about other case (actually it's the one which I had noticed > approximately at the time when culprit patch made the kernel). > > For some debugging purposes I have been using pin control table in board code. > > Since I would like to boot kernel on different systems I have some tables for > non-existing pin control device. Pin control framework returns -EPROBE_DEFER > when trying to probe device with attached table for wrong pin control. This is > fine, the problem is that *any* successfully probed device, which happens in > the deferred probe initcall will desynchronize existing counter. As a result -> > infinite loop. For the record, I didn't realize and didn't investigate that > time the issue and now I can confirm that this is a culprit which is fixed by > this patch. > > > > > > ---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; > > > } > > > > Thanks for the confirmations and pointers. I assume > > "linux,extcon-name" is a property that's obtained from ACPI? Because I > > couldn't find a relevant reference to it elsewhere in the kernel. > > Yes. > > > > > > ...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. > > > > Definitely agree. I'm not saying deferred probe is perfect. > > > > > Even if we fix for USB case, there is (and probably will be) others. > > > > > > > Most of the platform device code I've seen in systems with OF (device > > > > tree) add the child devices towards the end of the parent's probe > > > > function. > > I realized also that your fix won't work if we change extcon to be compiled in > and ULPI to be a module. So, any driver with two or more strict dependencies > one of which is satisfied and one is not will end up in this infinite loop. > > > > > > ...and since we had a successful probe, we got counter mismatch... > > > > > > > > > > [ 22.297490] driver_deferred_probe_trigger <<< 3 > > > > > [ 22.302074] platform dwc3.0.auto: deferred_probe_work_func 2 <<< counter 3 > > > > > > > > > > ...at the end we have a new counter and loop repeats again, see 22.198727... > > > > > > > > > > ---8<---8<--- > > > > > > > > > > Revert of the commit helps, but it is probably not helpful for the initially > > > > > found regression. Artem Bityutskiy suggested to use counter of the successful > > > > > probes instead. This fixes above mentioned case and shouldn't prevent driver > > > > > to reprobe deferred ones. > > > > > > > > > > Under "successful probe" we understand the state when a driver of the certain > > > > > device is being kept bound after deferred probe trigger cycle. For instance, > > > > > in the above mentioned case probing of tusb1210 is not successful because dwc3 > > > > > driver unbinds device dwc3.0.auto.ulpi. The atomic_dec() call is used to keep > > > > > track of this. The amount of bindings is always great than or equal to the > > > > > amount of unbindings as guaranteed by design of the driver binding mechanism. > > > > > > > > The unbindings count can increase for other unrelated drivers unbinding > > > > too. Wouldn't it? Seems a bit fragile and racy in a fashion similar to > > > > the issue the original patch was trying to fix. > > > > > > Yes, it's (unlikely) possible (*), but it will give one more iteration per such > > > case. It's definitely better than infinite loop. Do you agree? > > > > Sorry I wasn't being clear (I was in a rush). I'm saying this patch > > can reintroduce the bug where the deferred probe isn't triggered when > > it should be. > > I don't think so. If I'm not mistaken we still have one more cycle to trigger probe. > > > Let's take a simple execution flow. > > > > probe_okay is at 10. > > > > Thread-A > > really_probe(Device-A) > > local_probe_okay_count = 10 > > Device-A probe function is running... > > > > Thread-B > > really_probe(Device-B) > > Device-B probes successfully. > > probe_okay incremented to 11 > > And probe trigger task is called. It goes to the loop because counters are not the same.
Good point. But Device-A is still not in any of the deferred probed lists at this point AFAICU. So even the retriggered iteration can complete before Device-A is back on the deferred probe list?
> > > Thread-C > > Device-C (which had bound earlier) is unbound (say module is > > unloaded or a million other reasons). > > probe_okay is decremented to 10. > > > > Thread-A continues > > Device-A probe function returns -EPROBE_DEFER > > driver_deferred_probe_add_trigger() doesn't do anything because > > local_probe_okay_count == probe_okay > > But Device-A might have deferred probe waiting on Device-B. > > Device-A never probes. > > See above.
I think the key to fixing the original issue that commit 58b116bce136 fixes is to make sure the thread that has a device getting deferred is the one that re-triggers the deferred probe if it sees another device having probed successfully.
> > > *) It means during probe you have _intensive_ removing, of course you may keep > > > kernel busy with iterations, but it has no practical sense. DoS attacks more > > > effective in different ways. > > > > I wasn't worried about DoS attacks. More of a functional correctness > > issue what I explained above. > > > > Anyway, if your issue and similar issues can be handles in driver core > > in a clean way without breaking other cases, I don't have any problem > > with that. Just that, I think the current solution breaks other cases. > > > As an alternate solution, assuming "linux,extcon-name" is coming > > from some firmware, you might want to look into the fw_devlink > > feature. > > Let's forget about USB. Don't be fixed on it. It one of the particular case out > of others. I'm not going to comment USB particularities, sorry. It seems to me > as not related.
Ok, but just FYI, fw_devlink helps a lot with avoiding deferred probe issues. Has helped a ton with Android on ARM.
-Saravana
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