On Wed, 2012-11-28 at 00:41 +0100, Rafael J. Wysocki wrote: > 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. Right. This requires careful inspection and testing. As far as the locking is concerned, I am not keen on using fine grained locking for hot-plug. It is much simpler and solid if we serialize such operations. > 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? Umm... Sorry, I am not getting your point. The issue is that we need to be able to fail a request when memory range cannot be off-lined. Otherwise, we end up ejecting online memory range. Thanks, -Toshi -- To unsubscribe from this list: send the line "unsubscribe linux-acpi" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html