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From:   Saravana Kannan <saravanak@google.com>
Date:   Tue, 16 Feb 2021 09:07:26 -0800
Message-ID: <CAGETcx8nD7Ak8z7JEM1jUVdRRpUt=8BwGMix0ghv1QeDBLaGwA@mail.gmail.com>
Subject: Re: [PATCH] driver core: Fix double failed probing with fw_devlink=on
To:     Geert Uytterhoeven <geert@linux-m68k.org>
Cc:     "Rafael J. Wysocki" <rafael@kernel.org>,
        Geert Uytterhoeven <geert+renesas@glider.be>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Linux-Renesas <linux-renesas-soc@vger.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
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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:
> >
> > Hi Saravana,
> >
> > 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.

-Saravana