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

[Rafael J. Wysocki]

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.

I see two reasons for calling acpi_bus_hot_remove_device() for memory (correct
me if I'm wrong): (1) from the memhotplug driver's notify handler and (2) from
acpi_eject_store() which is exposed through sysfs.  If we disabled exposing
acpi_eject_store() for memory devices, then the only way would be from the
notify handler.  So I wonder if driver_unbind() shouldn't just uninstall the
notify handler for memory (so that memory eject events are simply dropped on
the floor after unbinding the driver)?

Rafael


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