On Wed, 2012-11-28 at 23:01 +0100, Rafael J. Wysocki wrote: > On Wednesday, November 28, 2012 02:40:09 PM Toshi Kani wrote: > > On Wed, 2012-11-28 at 22:40 +0100, Rafael J. Wysocki wrote: > > > On Wednesday, November 28, 2012 02:02:48 PM Toshi Kani wrote: > > > > > > > > > > > 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. > > > > > > > > > > Yes, this is the major one. The minor issue, however, is a race condition > > > > > between unbinding a driver from a device and removing the device if I > > > > > understand it correctly. Which will go away automatically if the device is > > > > > not removed in the first place. Or so I would think. :-) > > > > > > > > I see. I do not think whether or not the device is removed on eject > > > > makes any difference here. The issue is that after driver_unbind() is > > > > done, acpi_bus_hot_remove_device() no longer calls the ACPI memory > > > > driver (hence, it cannot fail in prepare_remove), and goes ahead to call > > > > _EJ0. If driver_unbind() did off-line the memory, this is OK. However, > > > > it cannot off-line kernel memory ranges. So, we basically need to > > > > either 1) serialize acpi_bus_hot_remove_device() and driver_unbind(), or > > > > 2) make acpi_bus_hot_remove_device() to fail if driver_unbind() is run > > > > during the operation. > > > > > > OK, I see the problem now. > > > > > > What exactly is triggering the driver_unbind() in this scenario? > > > > User can request driver_unbind() from sysfs as follows. I do not see > > much reason why user has to do for memory, though. > > > > echo "PNP0C80:XX" > /sys/bus/acpi/drivers/acpi_memhotplug/unbind > > This is wrong. Even if we want to permit user space to forcibly unbind > drivers from anything like this, we should at least check for some > situations in which it is plain dangerous. Like in this case. So I think > the above should fail unless we know that the driver won't be necessary > to handle hot-removal of memory. Well, we tried twice already... :) https://lkml.org/lkml/2012/11/16/649 > Alternatively, this may actually try to carry out the hot-removal and only > call driver_unbind() if that succeeds. Whichever is preferable, I'd say. Greg clarified in the above link that this interface is "unbind", not remove. Thanks, -Toshi -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>