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nova.virt.libvirt./driver.py:LibvirtDriver._live_migration() spawn a thread to execute _live_migration_operation(called after threadA).Original thread execute _live_migration_monitor (called after threadB).
Assignment statement inst_type=instance.flavor call nova/objects/instance.py:obj_load_attr in function _live_migration_operation.
Function _live_migration_monitor call func _live_migration_data_gb。Assignment statement ram_gb = instance.flavor.memory_mb * units.Mi / units.Gi also call nova/objects/instance.py:obj_load_attr. 
Function temporary_mutation is called by obj_load_attr. The mistack caused by the temporary_mutation is called by two threads simultaneously。
Time0: self._context[‘read.deleted’] is ‘no’
Time1: ThreadA called temporary_mutation, self._context[‘read.deleted’] is assigned a value of ‘yes’. Old value is ‘no’.
Time2: ThreadB called temporary_mutation, self._context[‘read.deleted’] is assigned a value of ‘yes’. Old value is ‘yes’.
Time3: ThreadA executing finally code of temporary_mutation, the value of  self._context[‘read.deleted’] is restored to ‘no’.
Time3: ThreadA executing finally code of temporary_mutation, the value of  self._context[‘read.deleted’] is restored to ‘yes’.
Result : Two calls to temporary_mutation cause the value of self._context[‘read.deleted’] to change from ‘no’ to ‘yes’. When Source host calling update_available_resource(ctxt) in _post_live_migration, Grabbing all instances assigned to this node will read deleted instances, which is time-consuming.

** Affects: nova
     Importance: Undecided
         Status: New

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A mistack caused by temporary_mutation reentry
https://bugs.launchpad.net/bugs/1941819
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