commit 6cf449bdd0d4beb95cf12311e7d2f8669e625fac
Author: Lee Yarwood <lyarwood@redhat.com>
Date:   Mon Sep 28 12:18:29 2020 +0100

    compute: Lock by instance.uuid lock during swap_volume
    
    The libvirt driver is currently the only virt driver implementing swap
    volume within Nova. While libvirt itself does support moving between
    multiple volumes attached to the same instance at the same time the
    current logic within the libvirt driver makes a call to
    virDomainGetXMLDesc that fails if there are active block jobs against
    any disk attached to the domain.
    
    This change simply uses an instance.uuid based lock in the compute layer
    to serialise requests to swap_volume to avoid this from being possible.
    
    Closes-Bug: #1896621
    Change-Id: Ic5ce2580e7638a47f1ffddb4edbb503bf490504c

diff --git a/nova/compute/manager.py b/nova/compute/manager.py
index 20f5f76..a81e458 100644
--- a/nova/compute/manager.py
+++ b/nova/compute/manager.py
@@ -7320,9 +7320,33 @@ class ComputeManager(manager.Manager):
     @wrap_instance_fault
     def swap_volume(self, context, old_volume_id, new_volume_id, instance,
                     new_attachment_id):
-        """Swap volume for an instance."""
-        context = context.elevated()
+        """Replace the old volume with the new volume within the active server
 
+        :param context: User request context
+        :param old_volume_id: Original volume id
+        :param new_volume_id: New volume id being swapped to
+        :param instance: Instance with original_volume_id attached
+        :param new_attachment_id: ID of the new attachment for new_volume_id
+        """
+        @utils.synchronized(instance.uuid)
+        def _do_locked_swap_volume(context, old_volume_id, new_volume_id,
+                                   instance, new_attachment_id):
+            self._do_swap_volume(context, old_volume_id, new_volume_id,
+                                 instance, new_attachment_id)
+        _do_locked_swap_volume(context, old_volume_id, new_volume_id, instance,
+                               new_attachment_id)
+
+    def _do_swap_volume(self, context, old_volume_id, new_volume_id,
+                        instance, new_attachment_id):
+        """Replace the old volume with the new volume within the active server
+
+        :param context: User request context
+        :param old_volume_id: Original volume id
+        :param new_volume_id: New volume id being swapped to
+        :param instance: Instance with original_volume_id attached
+        :param new_attachment_id: ID of the new attachment for new_volume_id
+        """
+        context = context.elevated()
         compute_utils.notify_about_volume_swap(
             context, instance, self.host,
             fields.NotificationPhase.START,