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    SubjectRe: [PATCH] driver core: Drop devices_kset_move_last() call from really_probe()
    On Tue, Jul 10, 2018 at 12:06 AM, Bjorn Helgaas <bhelgaas@google.com> wrote:
    > [+cc Kishon]
    >
    > On Mon, Jul 9, 2018 at 4:35 PM Rafael J. Wysocki <rafael@kernel.org> wrote:
    >>
    >> On Mon, Jul 9, 2018 at 3:57 PM, Bjorn Helgaas <bhelgaas@google.com> wrote:
    >> > On Fri, Jul 6, 2018 at 5:01 AM Rafael J. Wysocki <rjw@rjwysocki.net> wrote:
    >> >>
    >> >> From: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
    >> >>
    >> >> The devices_kset_move_last() call in really_probe() is a mistake
    >> >> as it may cause parents to follow children in the devices_kset list
    >> >> which then causes system shutdown to fail. Namely, if a device has
    >> >> children before really_probe() is called for it (which is not
    >> >> uncommon), that call will cause it to be reordered after the children
    >> >> in the devices_kset list and the ordering of that list will not
    >> >> reflect the correct device shutdown order.
    >> >>
    >> >> Also it causes the devices_kset list to be constantly reordered
    >> >> until all drivers have been probed which is totally pointless
    >> >> overhead in the majority of cases.
    >> >>
    >> >> For that reason, revert the really_probe() modifications made by
    >> >> commit 52cdbdd49853.
    >> >
    >> > I'm sure you've considered this, but I can't figure out whether this
    >> > patch will reintroduce the problem that was solved by 52cdbdd49853.
    >> > That patch updated two places: (1) really_probe(), the change you're
    >> > reverting here, and (2) device_move().
    >> >
    >> > device_move() is only called from 4-5 places, none of which look
    >> > related to the problem fixed by 52cdbdd49853, so it seems like that
    >> > problem was probably resolved by the hunk you're reverting.
    >>
    >> That's right, but I don't want to revert all of it. The other parts
    >> of it are kind of useful as they make the handling of the devices_kset
    >> list be consistent with the handling of dpm_list.
    >>
    >> The hunk I'm reverting, however, is completely off. It not only is
    >> incorrect (as per the above), but it also causes the devices_kset list
    >> and dpm_list to be handled differently.
    >
    > If I understand correctly, you are saying:
    >
    > - the 52cdbdd49853 really_probe() hunk fixed a problem, but

    It papered over a shutdown failure. Calling it a "fix" is an overstatement IMO.

    > - that hunk was the wrong fix for it, and
    > - this patch removes the wrong fix (and probably reintroduces the problem)
    >
    > If devices_kset is supposed to be ordered so children follow parents,
    > I agree the really_probe() hunk doesn't make much sense because the
    > parent/child relation is determined by the circuit design, not by the
    > probe order.

    Exactly.

    > It just seems like it's worth being clear that we're reintroducing the
    > problem fixed by 52cdbdd49853, so it needs to be solved a different
    > way.

    OK

    > Ideally that would be done before this patch so there's not a
    > regression, and this changelog could mention what's happening.

    Well, commit 52cdbdd49853 introduced a regression by itself, but that
    regression has only been reported recently.

    I don't really want to go into a discussion on which of the two
    regressions is more painful, but then IMO going back to the state from
    before commit 52cdbdd49853 is fair enough. Hence the patch.

    >> It had attempted to fix something, but it failed miserably at that.
    >
    > If you're saying that 52cdbdd49853 *tried* to fix a DRA7XX_evm reboot
    > problem, but in fact, it did not fix that problem, then I guess there
    > should be no issue with reverting that hunk.

    Again, it hid the reboot problem by changing the core in a way that
    led to a shutdown regression elsewhere.

    Also it looks like the platform(s) having that reboot issue do(es)n't
    really do system-wide suspend/resume, because that "fix" obviously
    doesn't help there.

    >> >> Fixes: 52cdbdd49853 (driver core: correct device's shutdown order)
    >> >> Link: https://lore.kernel.org/lkml/CAFgQCTt7VfqM=UyCnvNFxrSw8Z6cUtAi3HUwR4_xPAc03SgHjQ@mail.gmail.com/
    >> >> Reported-by: Pingfan Liu <kernelfans@gmail.com>
    >> >> Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
    >> >> ---
    >> >> drivers/base/dd.c | 8 --------
    >> >> 1 file changed, 8 deletions(-)
    >> >>
    >> >> Index: linux-pm/drivers/base/dd.c
    >> >> ===================================================================
    >> >> --- linux-pm.orig/drivers/base/dd.c
    >> >> +++ linux-pm/drivers/base/dd.c
    >> >> @@ -434,14 +434,6 @@ re_probe:
    >> >> goto probe_failed;
    >> >> }
    >> >>
    >> >> - /*
    >> >> - * Ensure devices are listed in devices_kset in correct order
    >> >> - * It's important to move Dev to the end of devices_kset before
    >> >> - * calling .probe, because it could be recursive and parent Dev
    >> >> - * should always go first
    >> >> - */
    >> >> - devices_kset_move_last(dev);
    >> >> -
    >> >> if (dev->bus->probe) {
    >> >> ret = dev->bus->probe(dev);
    >> >> if (ret)
    >> >>

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