Re: [PATCH] driver core: Fix double failed probing with fw_devlink=on

From: Saravana Kannan <saravanak@google.com>
To: Geert Uytterhoeven <geert@linux-m68k.org>
Cc: "Rafael J. Wysocki" <rafael@kernel.org>,
	Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	Linux-Renesas <linux-renesas-soc@vger.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH] driver core: Fix double failed probing with fw_devlink=on
Date: Wed, 7 Jul 2021 10:45:24 -0700	[thread overview]
Message-ID: <CAGETcx9JUqiA26-SdwCbnyj+w2FM648YmeJwMPZeX_5NCgdmfg@mail.gmail.com> (raw)
In-Reply-To: <CAMuHMdX-cZO-tsj6T9av79d_bELihBfFGmB1=F+6YRNmUBWs9g@mail.gmail.com>

On Wed, Jul 7, 2021 at 1:43 AM Geert Uytterhoeven <geert@linux-m68k.org> wrote:
>
> Hi Saravana,
>
> (going over old patch I still have in my local tree)
>
> On Tue, Feb 16, 2021 at 6:08 PM Saravana Kannan <saravanak@google.com> wrote:
> > On Mon, Feb 15, 2021 at 12:59 PM Saravana Kannan <saravanak@google.com> wrote:
> > > On Mon, Feb 15, 2021 at 11:08 AM Geert Uytterhoeven
> > > <geert@linux-m68k.org> wrote:
> > > > On Mon, Feb 15, 2021 at 7:27 PM Saravana Kannan <saravanak@google.com> wrote:
> > > > > On Mon, Feb 15, 2021 at 6:59 AM Rafael J. Wysocki <rafael@kernel.org> wrote:
> > > > > > On Mon, Feb 15, 2021 at 12:16 PM Geert Uytterhoeven
> > > > > > <geert+renesas@glider.be> wrote:
> > > > > > > With fw_devlink=permissive, devices are added to the deferred probe
> > > > > > > pending list if their driver's .probe() method returns -EPROBE_DEFER.
> > > > > > >
> > > > > > > With fw_devlink=on, devices are added to the deferred probe pending list
> > > > > > > if they are determined to be a consumer,
> > > > >
> > > > > If they are determined to be a consumer or if they are determined to
> > > > > have a supplier that hasn't probed yet?
> > > >
> > > > When the supplier has probed:
> > > >
> > > >     bus: 'platform': driver_probe_device: matched device
> > > > e6150000.clock-controller with driver renesas-cpg-mssr
> > > >     bus: 'platform': really_probe: probing driver renesas-cpg-mssr
> > > > with device e6150000.clock-controller
> > > >     PM: Added domain provider from /soc/clock-controller@e6150000
> > > >     driver: 'renesas-cpg-mssr': driver_bound: bound to device
> > > > 'e6150000.clock-controller'
> > > >     platform e6055800.gpio: Added to deferred list
> > > >     [...]
> > > >     platform e6020000.watchdog: Added to deferred list
> > > >     [...]
> > > >     platform fe000000.pcie: Added to deferred list
> > > >
> > > > > > > which happens before their
> > > > > > > driver's .probe() method is called.  If the actual probe fails later
> > > > > > > (real failure, not -EPROBE_DEFER), the device will still be on the
> > > > > > > deferred probe pending list, and it will be probed again when deferred
> > > > > > > probing kicks in, which is futile.
> > > > > > >
> > > > > > > Fix this by explicitly removing the device from the deferred probe
> > > > > > > pending list in case of probe failures.
> > > > > > >
> > > > > > > Fixes: e590474768f1cc04 ("driver core: Set fw_devlink=on by default")
> > > > > > > Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
> > > > > >
> > > > > > Good catch:
> > > > > >
> > > > > > Reviewed-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
> > > > >
> > > > > The issue is real and needs to be fixed. But I'm confused how this can
> > > > > happen. We won't even enter really_probe() if the driver isn't ready.
> > > > > We also won't get to run the driver's .probe() if the suppliers aren't
> > > > > ready. So how does the device get added to the deferred probe list
> > > > > before the driver is ready? Is this due to device_links_driver_bound()
> > > > > on the supplier?
> > > > >
> > > > > Can you give a more detailed step by step on the case you are hitting?
> > > >
> > > > The device is added to the list due to device_links_driver_bound()
> > > > calling driver_deferred_probe_add() on all consumer devices.
> > >
> > > Thanks for the explanation. Maybe add more details like this to the
> > > commit text or in the code?
> > >
> > > For the code:
> > > Reviewed-by: Saravana Kanna <saravanak@google.com>
> >
> > Ugh... I just realized that I might have to give this a Nak because of
> > bad locking in deferred_probe_work_func(). The unlock/lock inside the
> > loop is a terrible hack. If we add this patch, we can end up modifying
> > a linked list while it's being traversed and cause a crash or busy
> > loop (you'll accidentally end up on an "empty list"). I ran into a
> > similar issue during one of my unrelated refactors.
>
> Turns out the issue I was seeing went away due to commit
> f2db85b64f0af141 ("driver core: Avoid pointless deferred probe
> attempts"), so there is no need to apply this patch.
>

Yay! That was the goal :) I'm assuming it wasn't ever applied.

-Saravana