linux-kernel - Re: [PATCH v1 1/3] driver core: Don't do deferred probe in parallel with kernel

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Date:   Tue, 7 Jul 2020 14:54:21 -0700
From:   Saravana Kannan <saravanak@...gle.com>
To:     "Rafael J. Wysocki" <rafael@...nel.org>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Geert Uytterhoeven <geert@...ux-m68k.org>,
        "Cc: Android Kernel" <kernel-team@...roid.com>
Subject: Re: [PATCH v1 1/3] driver core: Don't do deferred probe in parallel
 with kernel_init thread

On Tue, Jul 7, 2020 at 8:59 AM Rafael J. Wysocki <rafael@...nel.org> wrote:
>
> On Wed, Jul 1, 2020 at 9:43 PM Saravana Kannan <saravanak@...gle.com> wrote:
> >
> > The current deferred probe implementation can mess up suspend/resume
> > ordering if deferred probe thread is kicked off in parallel with the
> > main initcall thread (kernel_init thread) [1].
> >
> > For example:
> >
> > Say device-B is a consumer of device-A.
> >
> > Initcall thread                                 Deferred probe thread
> > ===============                                 =====================
> > 1. device-A is added.
> > 2. device-B is added.
> > 3. dpm_list is now [device-A, device-B].
> > 4. driver-A defers probe of device-A.
> >                                                 5. device-A is moved to
> >                                                    end of dpm_list
> >                                                 6. dpm_list is now
> >                                                    [device-B, device-A]
> > 7. driver-B is registereed and probes device-B.
> > 8. dpm_list stays as [device-B, device-A].
> >
> > The reverse order of dpm_list is used for suspend. So in this case
> > device-A would incorrectly get suspended before device-B.
> >
> > Commit 716a7a259690 ("driver core: fw_devlink: Add support for batching
> > fwnode parsing") kicked off the deferred probe thread early during boot
> > to run in parallel with the initcall thread and caused suspend/resume
> > regressions.  This patch removes the parallel run of the deferred probe
> > thread to avoid the suspend/resume regressions.
> >
> > [1] - https://lore.kernel.org/lkml/CAGETcx8W96KAw-d_siTX4qHB_-7ddk0miYRDQeHE6E0_8qx-6Q@mail.gmail.com/
> >
> > Fixes: 716a7a259690 ("driver core: fw_devlink: Add support for batching fwnode parsing")
> > Signed-off-by: Saravana Kannan <saravanak@...gle.com>
> > ---
> >  drivers/base/base.h | 1 -
> >  drivers/base/core.c | 1 -
> >  drivers/base/dd.c   | 5 -----
> >  3 files changed, 7 deletions(-)
> >
> > diff --git a/drivers/base/base.h b/drivers/base/base.h
> > index 95c22c0f9036..40fb069a8a7e 100644
> > --- a/drivers/base/base.h
> > +++ b/drivers/base/base.h
> > @@ -153,7 +153,6 @@ extern char *make_class_name(const char *name, struct kobject *kobj);
> >  extern int devres_release_all(struct device *dev);
> >  extern void device_block_probing(void);
> >  extern void device_unblock_probing(void);
> > -extern void driver_deferred_probe_force_trigger(void);
> >
> >  /* /sys/devices directory */
> >  extern struct kset *devices_kset;
> > diff --git a/drivers/base/core.c b/drivers/base/core.c
> > index 67d39a90b45c..35cc9896eb9e 100644
> > --- a/drivers/base/core.c
> > +++ b/drivers/base/core.c
> > @@ -1323,7 +1323,6 @@ void fw_devlink_resume(void)
> >                 goto out;
> >
> >         device_link_add_missing_supplier_links();
> > -       driver_deferred_probe_force_trigger();
>
> So it looks like this was not needed after all.
>
> What was the role of it then?

It's needed to avoid delaying all probe till late_initcall. Patch 3/3
has more details.

-Saravana