linux-kernel - Re: [PATCHv3 0/4] drivers/base: bugfix for supplier<-consumer ordering in device

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Date:   Thu, 5 Jul 2018 10:44:57 +0800
From:   Pingfan Liu <kernelfans@...il.com>
To:     rjw@...ysocki.net
Cc:     linux-kernel@...r.kernel.org,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Grygorii Strashko <grygorii.strashko@...com>,
        Christoph Hellwig <hch@...radead.org>,
        Bjorn Helgaas <helgaas@...nel.org>,
        Dave Young <dyoung@...hat.com>, linux-pci@...r.kernel.org,
        linuxppc-dev@...ts.ozlabs.org, linux-pm@...r.kernel.org
Subject: Re: [PATCHv3 0/4] drivers/base: bugfix for supplier<-consumer
 ordering in device_kset

On Wed, Jul 4, 2018 at 6:23 PM Rafael J. Wysocki <rjw@...ysocki.net> wrote:
>
> On Wednesday, July 4, 2018 4:47:07 AM CEST Pingfan Liu wrote:
> > On Tue, Jul 3, 2018 at 10:36 PM Rafael J. Wysocki <rjw@...ysocki.net> wrote:
> > >
> > > On Tuesday, July 3, 2018 8:50:38 AM CEST Pingfan Liu wrote:
> > > > commit 52cdbdd49853 ("driver core: correct device's shutdown order")
> > > > places an assumption of supplier<-consumer order on the process of probe.
> > > > But it turns out to break down the parent <- child order in some scene.
> > > > E.g in pci, a bridge is enabled by pci core, and behind it, the devices
> > > > have been probed. Then comes the bridge's module, which enables extra
> > > > feature(such as hotplug) on this bridge.
> > >
> > > So what *exactly* does happen in that case?
> > >
> > I saw the  shpc_probe() is called on the bridge, although the probing
> > failed on that bare-metal. But if it success, then it will enable the
> > hotplug feature on the bridge.
>
> I don't understand what you are saying here, sorry.
>
On the system, I observe the following:
[    2.114986] devices_kset: Moving 0004:00:00.0 to end of list
<---pcie port drive's probe, but it failed
[    2.115192] devices_kset: Moving 0004:01:00.0 to end of list
[    2.115591] devices_kset: Moving 0004:02:02.0 to end of list
[    2.115923] devices_kset: Moving 0004:02:0a.0 to end of list
[    2.116141] devices_kset: Moving 0004:02:0b.0 to end of list
[    2.116358] devices_kset: Moving 0004:02:0c.0 to end of list
[    3.181860] devices_kset: Moving 0004:03:00.0 to end of list
<---the ata disk controller which sits behind the bridge
[   10.267081] devices_kset: Moving 0004:00:00.0 to end of list
 <---shpc_probe() on this bridge, failed too.

As you can the the parent device "0004:00:00.0" is moved twice, and
finally, it is after the "0004:03:00.0", this will break the
"parent<-child" order in devices_kset. This is caused by the code
really_probe()->devices_kset_move_last(). Apparently, it makes
assumption that child device's probing comes after its parent's. But
it does not stand up in the case.

> device_reorder_to_tail() walks the entire device hierarchy below the target
> and moves all of the children in there *after* their parents.
>
As described, the bug is not related with device_reorder_to_tail(), it
is related with really_probe()->devices_kset_move_last(). So [2/4]
uses different method to achieve the "parent<-child" and
"supplier<-consumer" order. The [3/4] clean up some code in
device_reorder_to_tail(), since I need to revert the commit.

> How can it break "the parent <- child order" then?
>
As described, it does not, just not be in use any longer.

Thanks and regards,
Pingfan