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SubjectRe: [BUGFIX v2 2/4] ACPI, DOCK: resolve possible deadlock scenarios
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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?

Rafael


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


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