RE: [Update][PATCH] ACPI / hotplug: Fix concurrency issues and memory leaks

Moore, Robert
2013-02-14T23:45:47Z

> -----Original Message-----
> From: Rafael J. Wysocki [mailto:rjw@sisk.pl]
> Sent: Thursday, February 14, 2013 12:59 PM
> To: Moore, Robert
> Cc: Toshi Kani; ACPI Devel Maling List; LKML; Bjorn Helgaas; Jiang Liu;
> Yinghai Lu; Yasuaki Ishimatsu; Myron Stowe; linux-pci@vger.kernel.org
> Subject: Re: [Update][PATCH] ACPI / hotplug: Fix concurrency issues and
> memory leaks
>
> On Thursday, February 14, 2013 08:45:14 PM Moore, Robert wrote:
> >
> > > -----Original Message-----
> > > From: Rafael J. Wysocki [mailto:rjw@sisk.pl]
> > > Sent: Thursday, February 14, 2013 4:04 AM
> > > To: Moore, Robert
> > > Cc: Toshi Kani; ACPI Devel Maling List; LKML; Bjorn Helgaas; Jiang
> > > Liu; Yinghai Lu; Yasuaki Ishimatsu; Myron Stowe;
> > > linux-pci@vger.kernel.org
> > > Subject: Re: [Update][PATCH] ACPI / hotplug: Fix concurrency issues
> > > and memory leaks
> > >
> > > On Thursday, February 14, 2013 02:31:22 AM Moore, Robert wrote:
> > > > > > > I thought about that, but actually there's no guarantee that
> > > > > > > the handle will be valid after _EJ0 as far as I can say. So
> > > > > > > the race condition is going to be there anyway and using
> > > > > > > struct acpi_device just makes it easier to avoid it.
> > > > > >
> > > > > > In theory, yes, a stale handle could be a problem, if _EJ0
> > > > > > performs unload table and if ACPICA frees up its internal data
> > > > > > structure pointed by the handle as a result. But we should
> > > > > > not see such issue now since we do not support dynamic ACPI
> > > > > > namespace
> > > yet.
> > > > >
> > > > > I'm waiting for information from Bob about that. If we can
> > > > > assume ACPI handles to be always valid, that will simplify
> > > > > things quite a
> > > bit.
> > > >
> > > > If a table is unloaded, all the namespace nodes for that table are
> > > > removed from the namespace, and thus any ACPI_HANDLE pointers go
> > > > stale
> > > and invalid.
> > >
> > > OK, thanks!
> > >
> > > To me this means that we cannot assume a handle to stay valid
> > > between a notify handler and acpi_bus_hot_remove_device() run from a
> workqueue.
> > >
> > > Is there a mechanism in ACPICA to ensure that a handle won't become
> > > stale while a notify handler is running for it or is the OS
> > > responsible for ensuring that
> > > _EJ0 won't be run in parallel with notify handlers for device
> > > objects being ejected?
> > >
> >
> > It is up to the host.
>
> I was afraid that that might be the case. :-)
>
> So far the (Linux) host has been happily ignoring that potential problem,
> so I guess it can still be ignored for a while, although we'll need to
> address it eventually at one point.

I would think it should be fairly simple to setup a mechanism to either tell the driver or for the driver to figure it out -- such that the driver knows that all handles associated with the device are now invalid. Another way to look at it is that when the device is re-installed, the driver should reinitialize such that it obtains new handles for the devices and subobjects in question.

Bob

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