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    SubjectRe: [RFC PATCH v3 3/3] acpi_memhotplug: Allow eject to proceed on rebind scenario
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    On Wednesday, November 28, 2012 09:01:13 AM Toshi Kani wrote:
    > 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.

    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. :-)

    Thanks,
    Rafael


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


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