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Date:   Thu, 26 Mar 2020 17:39:53 +0100
From:   Greg KH <gregkh@linuxfoundation.org>
To:     Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Cc:     Grant Likely <grant.likely@arm.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: <20200326163953.GA1551380@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>
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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.

> 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.

thanks,

greg k-h