linux-kernel - Re: [PATCH] driver core: Drop devices_kset_move_last() call from really

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Date:   Mon, 9 Jul 2018 23:35:14 +0200
From:   "Rafael J. Wysocki" <rafael@...nel.org>
To:     Bjorn Helgaas <bhelgaas@...gle.com>
Cc:     "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Pingfan Liu <kernelfans@...il.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Grygorii Strashko <grygorii.strashko@...com>,
        Christoph Hellwig <hch@...radead.org>,
        Bjorn Helgaas <helgaas@...nel.org>,
        Dave Young <dyoung@...hat.com>,
        Linux PCI <linux-pci@...r.kernel.org>,
        Lukas Wunner <lukas@...ner.de>,
        Linux PM list <linux-pm@...r.kernel.org>
Subject: Re: [PATCH] driver core: Drop devices_kset_move_last() call from really_probe()

On Mon, Jul 9, 2018 at 3:57 PM, Bjorn Helgaas <bhelgaas@...gle.com> wrote:
> On Fri, Jul 6, 2018 at 5:01 AM Rafael J. Wysocki <rjw@...ysocki.net> wrote:
>>
>> From: Rafael J. Wysocki <rafael.j.wysocki@...el.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.

It had attempted to fix something, but it failed miserably at that.

>> 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@...il.com>
>> Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@...el.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)
>>