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
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