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    Subject[PATCH 5.0 136/137] PCI: hv: Add pci_destroy_slot() in pci_devices_present_work(), if necessary
    Date
    From: Dexuan Cui <decui@microsoft.com>

    commit 340d455699400f2c2c0f9b3f703ade3085cdb501 upstream.

    When we hot-remove a device, usually the host sends us a PCI_EJECT message,
    and a PCI_BUS_RELATIONS message with bus_rel->device_count == 0.

    When we execute the quick hot-add/hot-remove test, the host may not send
    us the PCI_EJECT message if the guest has not fully finished the
    initialization by sending the PCI_RESOURCES_ASSIGNED* message to the
    host, so it's potentially unsafe to only depend on the
    pci_destroy_slot() in hv_eject_device_work() because the code path

    create_root_hv_pci_bus()
    -> hv_pci_assign_slots()

    is not called in this case. Note: in this case, the host still sends the
    guest a PCI_BUS_RELATIONS message with bus_rel->device_count == 0.

    In the quick hot-add/hot-remove test, we can have such a race before
    the code path

    pci_devices_present_work()
    -> new_pcichild_device()

    adds the new device into the hbus->children list, we may have already
    received the PCI_EJECT message, and since the tasklet handler

    hv_pci_onchannelcallback()

    may fail to find the "hpdev" by calling

    get_pcichild_wslot(hbus, dev_message->wslot.slot)

    hv_pci_eject_device() is not called; Later, by continuing execution

    create_root_hv_pci_bus()
    -> hv_pci_assign_slots()

    creates the slot and the PCI_BUS_RELATIONS message with
    bus_rel->device_count == 0 removes the device from hbus->children, and
    we end up being unable to remove the slot in

    hv_pci_remove()
    -> hv_pci_remove_slots()

    Remove the slot in pci_devices_present_work() when the device
    is removed to address this race.

    pci_devices_present_work() and hv_eject_device_work() run in the
    singled-threaded hbus->wq, so there is not a double-remove issue for the
    slot.

    We cannot offload hv_pci_eject_device() from hv_pci_onchannelcallback()
    to the workqueue, because we need the hv_pci_onchannelcallback()
    synchronously call hv_pci_eject_device() to poll the channel
    ringbuffer to work around the "hangs in hv_compose_msi_msg()" issue
    fixed in commit de0aa7b2f97d ("PCI: hv: Fix 2 hang issues in
    hv_compose_msi_msg()")

    Fixes: a15f2c08c708 ("PCI: hv: support reporting serial number as slot information")
    Signed-off-by: Dexuan Cui <decui@microsoft.com>
    [lorenzo.pieralisi@arm.com: rewritten commit log]
    Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
    Reviewed-by: Stephen Hemminger <stephen@networkplumber.org>
    Reviewed-by: Michael Kelley <mikelley@microsoft.com>
    Cc: stable@vger.kernel.org
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

    ---
    drivers/pci/controller/pci-hyperv.c | 4 ++++
    1 file changed, 4 insertions(+)

    --- a/drivers/pci/controller/pci-hyperv.c
    +++ b/drivers/pci/controller/pci-hyperv.c
    @@ -1781,6 +1781,10 @@ static void pci_devices_present_work(str
    hpdev = list_first_entry(&removed, struct hv_pci_dev,
    list_entry);
    list_del(&hpdev->list_entry);
    +
    + if (hpdev->pci_slot)
    + pci_destroy_slot(hpdev->pci_slot);
    +
    put_pcichild(hpdev);
    }


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