Re: [RFC PATCH v3 3/3] acpi_memhotplug: Allow eject to proceed on rebind scenario

[Rafael J. Wysocki]

On Tuesday, November 27, 2012 03:03:47 PM Toshi Kani wrote:
> On Tue, 2012-11-27 at 19:32 +0100, Vasilis Liaskovitis wrote:
> > On Mon, Nov 26, 2012 at 05:19:01PM -0700, Toshi Kani wrote:
> > > > >> Consider the following sequence of operations for a hotplugged memory
> > > > >> device:
> > > > >>
> > > > >> 1. echo "PNP0C80:XX" > /sys/bus/acpi/drivers/acpi_memhotplug/unbind
> > > > >> 2. echo 1 >/sys/bus/pci/devices/PNP0C80:XX/eject
> > > > >>
> > > > >> If we don't offline/remove the memory, we have no chance to do it in
> > > > >> step 2. After
> > > > >> step2, the memory is used by the kernel, but we have powered off it. It
> > > > >> is very
> > > > >> dangerous.
> > > > > 
> > > > > How does power-off happen after unbind? acpi_eject_store checks for existing
> > > > > driver before taking any action:
> > > > > 
> > > > > #ifndef FORCE_EJECT
> > > > > 	if (acpi_device->driver == NULL) {
> > > > > 		ret = -ENODEV;
> > > > > 		goto err;
> > > > > 	}
> > > > > #endif
> > > > > 
> > > > > FORCE_EJECT is not defined afaict, so the function returns without scheduling
> > > > > acpi_bus_hot_remove_device. Is there another code path that calls power-off?
> > > > 
> > > > Consider the following case:
> > > > 
> > > > We hotremove the memory device by SCI and unbind it from the driver at the same time:
> > > > 
> > > > CPUa                                                  CPUb
> > > > acpi_memory_device_notify()
> > > >                                        unbind it from the driver
> > > >     acpi_bus_hot_remove_device()
> > > 
> > > Can we make acpi_bus_remove() to fail if a given acpi_device is not
> > > bound with a driver?  If so, can we make the unbind operation to perform
> > > unbind only?
> > 
> > acpi_bus_remove_device could check if the driver is present, and return -ENODEV
> > if it's not present (dev->driver == NULL).
> > 
> > But there can still be a race between an eject and an unbind operation happening
> > simultaneously. This seems like a general problem to me i.e. not specific to an
> > acpi memory device. How do we ensure an eject does not race with a driver unbind
> > for other acpi devices?
> > 
> > Is there a per-device lock in acpi-core or device-core that can prevent this from
> > happening? Driver core does a device_lock(dev) on all operations, but this is
> > probably not grabbed on SCI-initiated acpi ejects.
> 
> Since driver_unbind() calls device_lock(dev->parent) before calling
> device_release_driver(), I am wondering if we can call
> device_lock(dev->dev->parent) at the beginning of acpi_bus_remove()
> (i.e. before calling pre_remove) and fails if dev->driver is NULL.  The
> parent lock is otherwise released after device_release_driver() is done.

I would be careful.  You may introduce some subtle locking-related issues
this way.

Besides, there may be an alternative approach to all this.  For example,
what if we don't remove struct device objects on eject?  The ACPI handles
associated with them don't go away in that case after all, do they?

Rafael


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