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Date:   Fri, 27 Mar 2020 09:03:23 +0100
From:   Greg KH <gregkh@linuxfoundation.org>
To:     Grant Likely <grant.likely@arm.com>
Cc:     Andy Shevchenko <andriy.shevchenko@linux.intel.com>,
        Saravana Kannan <saravanak@google.com>, a.hajda@samsung.com,
        artem.bityutskiy@linux.intel.com, balbi@kernel.org,
        broonie@kernel.org, fntoth@gmail.com, linux-kernel@vger.kernel.org,
        linux-pm@vger.kernel.org, peter.ujfalusi@ti.com, rafael@kernel.org,
        kernel-team@android.com, nd <nd@arm.com>
Subject: Re: [PATCH v3] driver core: Break infinite loop when deferred probe
 can't be satisfied
Message-ID: <20200327080323.GA1627562@kroah.com>
References: <20200324175719.62496-1-andriy.shevchenko@linux.intel.com>
 <20200325032901.29551-1-saravanak@google.com>
 <20200325125120.GX1922688@smile.fi.intel.com>
 <295d25de-f01e-26de-02d6-1ac0c149d828@arm.com>
 <20200326163110.GD1922688@smile.fi.intel.com>
 <20200326163953.GA1551380@kroah.com>
 <9350ec52-345f-5c3c-f175-4e256699b9cf@arm.com>
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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