Re: [PATCH v3] driver core: Break infinite loop when deferred probe can't be satisfied

[Grant Likely]

On 26/03/2020 11:54, Andy Shevchenko 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@xxxxxxxxxxxxxxx> 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@xxxxxxxxxxxxxxx> 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.

Specific code path please?

g.