linux-kernel - Re: [BUGFIX v2 2/4] ACPI, DOCK: resolve possible deadlock scenarios

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Date:	Mon, 17 Jun 2013 14:54:49 +0200
From:	"Rafael J. Wysocki" <rjw@...k.pl>
To:	Jiang Liu <liuj97@...il.com>
Cc:	Bjorn Helgaas <bhelgaas@...gle.com>,
	Yinghai Lu <yinghai@...nel.org>,
	"Alexander E . Patrakov" <patrakov@...il.com>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Yijing Wang <wangyijing@...wei.com>,
	linux-acpi@...r.kernel.org, linux-pci@...r.kernel.org,
	linux-kernel@...r.kernel.org, Len Brown <lenb@...nel.org>,
	stable@...r.kernel.org, Jiang Liu <jiang.liu@...wei.com>
Subject: Re: [BUGFIX v2 2/4] ACPI, DOCK: resolve possible deadlock scenarios

On Monday, June 17, 2013 01:39:04 PM Rafael J. Wysocki wrote:
> On Monday, June 17, 2013 01:01:51 AM Jiang Liu wrote:
> > On 06/16/2013 05:20 AM, Rafael J. Wysocki wrote:
> > > On Saturday, June 15, 2013 10:17:42 PM Rafael J. Wysocki wrote:
> > >> On Saturday, June 15, 2013 09:44:28 AM Jiang Liu wrote:
> > [...]
> > >> When it returns from unregister_hotplug_dock_device(), nothing prevents it
> > >> from accessing whatever it wants, because ds->hp_lock is not used outside
> > >> of the add/del and hotplug_dock_devices().  So, the actual role of
> > >> ds->hp_lock (not the one that it is supposed to play, but the real one)
> > >> is to prevent addition/deletion from happening when hotplug_dock_devices()
> > >> is running.  [Yes, it does protect the list, but since the list is in fact
> > >> unnecessary, that doesn't matter.]
> > >>
> > >>> If we simply use a flag to mark presence of registered callback, we 
> > >>> can't achieve the second goal.
> > >>
> > >> I don't mean using the flag *alone*.
> > >>
> > >>> Take the sony laptop as an example. It has several PCI 
> > >>> hotplug
> > >>> slot associated with the dock station:
> > >>> [   28.829316] acpiphp_glue: _handle_hotplug_event_func: Bus check 
> > >>> notify on \_SB_.PCI0.RP07.LPMB
> > >>> [   30.174964] acpiphp_glue: _handle_hotplug_event_func: Bus check 
> > >>> notify on \_SB_.PCI0.RP07.LPMB.LPM0
> > >>> [   30.174973] acpiphp_glue: _handle_hotplug_event_func: Bus check 
> > >>> notify on \_SB_.PCI0.RP07.LPMB.LPM1
> > >>> [   30.174979] acpiphp_glue: _handle_hotplug_event_func: Bus check 
> > >>> notify on \_SB_.PCI0.RP07.LPMB.LPM2
> > >>> [   30.174985] acpiphp_glue: _handle_hotplug_event_func: Bus check 
> > >>> notify on \_SB_.PCI0.RP07.LPMB.LPM2.LPRI.LPR0.GFXA
> > >>> [   30.175020] acpiphp_glue: _handle_hotplug_event_func: Bus check 
> > >>> notify on \_SB_.PCI0.RP07.LPMB.LPM2.LPRI.LPR0.GHDA
> > >>> [   30.175040] acpiphp_glue: _handle_hotplug_event_func: Bus check 
> > >>> notify on \_SB_.PCI0.RP07.LPMB.LPM2.LPRI.LPR1.LPCI.LPC0.DLAN
> > >>> [   30.175050] acpiphp_glue: _handle_hotplug_event_func: Bus check 
> > >>> notify on \_SB_.PCI0.RP07.LPMB.LPM2.LPRI.LPR1.LPCI.LPC1.DODD
> > >>> [   30.175060] acpiphp_glue: _handle_hotplug_event_func: Bus check 
> > >>> notify on \_SB_.PCI0.RP07.LPMB.LPM2.LPRI.LPR1.LPCI.LPC2.DUSB
> > >>>
> > >>> So it still has some race windows if we undock the station while 
> > >>> repeatedly rescanning/removing
> > >>> the PCI bus for \_SB_.PCI0.RP07.LPMB.LPM0 through sysfs interfaces.
> > > 
> > > Which sysfs interfaces do you mean, by the way?
> > > 
> > > If you mean "eject", then it takes acpi_scan_lock and hotplug_dock_devices()
> > > should always be run under acpi_scan_lock too.  It isn't at the moment,
> > > because write_undock() doesn't take acpi_scan_lock(), but this is an obvious
> > > bug (so I'm going to send a patch to fix it in a while).
> > > 
> > > With that bug fixed, the possible race between acpi_eject_store() and
> > > hotplug_dock_devices() should be prevented from happening, so perhaps we're
> > > worrying about something that cannot happen?
> > Hi Rafael,
> > 	I mean the "remove" method of each PCI device, and the "power" method
> > of PCI hotplug slot here.
> > 	These methods may be used to remove P2P bridges with associated ACPIPHP
> > hotplug slots, which in turn will cause invoking of
> > unregister_hotplug_dock_device().
> > 	So theoretical we may trigger the bug by undocking while repeatedly
> > adding/removing P2P bridges with ACPIPHP hotplug slot through PCI
> > "rescan" and "remove" sysfs interface,
> 
> Why don't we make these things take acpi_scan_lock upfront, then?

Or perhaps (and maybe better) why don't we replace ds->hp_lock by another
lock that will be acquired upper in the call chain so that
dock_add_hotplug_device(), dock_del_hotplug_device(), hotplug_dock_devices()
and dock_event() are all guaranteed to be called under that lock?

Rafael


-- 
I speak only for myself.
Rafael J. Wysocki, Intel Open Source Technology Center.
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