s390x/pci: Unregister listeners before destroying IOMMU address space

s390x/pci: Unregister listeners before destroying IOMMU address space

Hot-unplugging a vfio-pci device on s390x causes a QEMU crash:

qemu-system-s390x: ../softmmu/memory.c:2772:
 do_address_space_destroy: Assertion `QTAILQ_EMPTY(&as->listeners)' failed.

In s390, the IOMMU address space is freed during device unplug but the
associated vfio-pci device may not yet be finalized and therefore may
still have a listener registered to the IOMMU address space.

Commit a2166410ad74 ("spapr_pci: Unregister listeners before destroying
the IOMMU address space") previously resolved this issue for spapr_pci.
We are now seeing this in s390x; it would seem the possibility for this
issue was already present but based on a bisect commit 2d24a6466154
("device-core: use RCU for list of children of a bus") has now changed
the timing such that it is now readily reproducible.

Add logic to ensure listeners are removed before destroying the address
space.

Reported-by: Niklas Schnelle <schnelle@linux.ibm.com>
Signed-off-by: Matthew Rosato <mjrosato@linux.ibm.com>
Tested-by: Niklas Schnelle <schnelle@linux.ibm.com>
Reviewed-by: Pierre Morel <pmorel@linux.ibm.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Message-Id: <1605562955-21152-1-git-send-email-mjrosato@linux.ibm.com>
Signed-off-by: Cornelia Huck <cohuck@redhat.com>