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:

quoted

quoted

quoted

quoted

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.

Thanks,
-Toshi

--
To unsubscribe, send a message with 'unsubscribe linux-mm' in
the body to majordomo@kvack.org.  For more info on Linux MM,
see: http://www.linux-mm.org/ .
Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>