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One variant is race between zone_postload and resume_iteration, and the second variant is race between zone_postload and receive_secure_db.

WARNING: ThreadSanitizer: lock-order-inversion (potential deadlock)
  Cycle in lock order graph: M1 (0x000000000000) => M2 (0x000000000000) => M1

  Mutex M2 acquired here while holding mutex M1 in thread T1:
    #0 pthread_rwlock_wrlock <null>
    #1 isc_rwlock_lock lib/isc/rwlock.c:54:4
    #2 zone_postload lib/dns/zone.c:4908:2
    #3 receive_secure_db lib/dns/zone.c:15645:11
    #4 dispatch lib/isc/task.c:1143:7
    #5 run lib/isc/task.c:1328:2

  Mutex M1 previously acquired by the same thread here:
    #0 pthread_rwlock_rdlock <null>
    #1 isc_rwlock_lock lib/isc/rwlock.c:50:3
    #2 resume_iteration lib/dns/rbtdb.c:8946:2
    #3 dbiterator_first lib/dns/rbtdb.c:8993:3
    #4 dns_dbiterator_first lib/dns/dbiterator.c:44:10
    #5 receive_secure_db lib/dns/zone.c:15575:16
    #6 dispatch lib/isc/task.c:1143:7
    #7 run lib/isc/task.c:1328:2

  Mutex M1 acquired here while holding mutex M2 in thread T2:
    #0 pthread_rwlock_rdlock <null>
    #1 isc_rwlock_lock lib/isc/rwlock.c:50:3
    #2 findnodeintree lib/dns/rbtdb.c:2742:2
    #3 findnode lib/dns/rbtdb.c:2804:10
    #4 dns_db_findnode lib/dns/db.c:447:11
    #5 zone_get_from_db lib/dns/zone.c:5373:11
    #6 get_raw_serial lib/dns/zone.c:2398:12
    #7 zone_gotwritehandle lib/dns/zone.c:2435:4
    #8 dispatch lib/isc/task.c:1143:7
    #9 run lib/isc/task.c:1328:2

  Mutex M2 previously acquired by the same thread here:
    #0 pthread_rwlock_rdlock <null>
    #1 isc_rwlock_lock lib/isc/rwlock.c:50:3
    #2 zone_gotwritehandle lib/dns/zone.c:2428:2
    #3 dispatch lib/isc/task.c:1143:7
    #4 run lib/isc/task.c:1328:2

  Thread T1 (running) created by main thread at:
    #0 pthread_create <null>
    #1 isc_thread_create lib/isc/pthreads/thread.c:75:8
    #2 isc_taskmgr_create lib/isc/task.c:1419:3
    #3 create_managers bin/named/./main.c:902:11
    #4 setup bin/named/./main.c:1235:11
    #5 main bin/named/./main.c:1515:2

  Thread T2 (running) created by main thread at:
    #0 pthread_create <null>
    #1 isc_thread_create lib/isc/pthreads/thread.c:75:8
    #2 isc_taskmgr_create lib/isc/task.c:1419:3
    #3 create_managers bin/named/./main.c:902:11
    #4 setup bin/named/./main.c:1235:11
    #5 main bin/named/./main.c:1515:2

SUMMARY: ThreadSanitizer: lock-order-inversion (potential deadlock) in pthread_rwlock_wrlock

WARNING: ThreadSanitizer: lock-order-inversion (potential deadlock)
  Cycle in lock order graph: M1 (0x000000000001) => M2 (0x000000000002) => M1

  Mutex M2 acquired here while holding mutex M1 in thread T1:
    #0 pthread_rwlock_wrlock <null>
    #1 isc_rwlock_lock lib/isc/rwlock.c:54:4
    #2 zone_postload lib/dns/zone.c:4908:2
    #3 receive_secure_db lib/dns/zone.c:15645:11
    #4 dispatch lib/isc/task.c:1143:7
    #5 run lib/isc/task.c:1328:2

  Mutex M1 previously acquired by the same thread here:
    #0 pthread_mutex_lock <null>
    #1 receive_secure_db lib/dns/zone.c:15643:2
    #2 dispatch lib/isc/task.c:1143:7
    #3 run lib/isc/task.c:1328:2

  Mutex M1 acquired here while holding mutex M2 in thread T1:
    #0 pthread_mutex_lock <null>
    #1 get_raw_serial lib/dns/zone.c:2396:2
    #2 zone_gotwritehandle lib/dns/zone.c:2435:4
    #3 dispatch lib/isc/task.c:1143:7
    #4 run lib/isc/task.c:1328:2

  Mutex M2 previously acquired by the same thread here:
    #0 pthread_rwlock_rdlock <null>
    #1 isc_rwlock_lock lib/isc/rwlock.c:50:3
    #2 zone_gotwritehandle lib/dns/zone.c:2428:2
    #3 dispatch lib/isc/task.c:1143:7
    #4 run lib/isc/task.c:1328:2

  Thread T1 (running) created by main thread at:
    #0 pthread_create <null>
    #1 isc_thread_create lib/isc/pthreads/thread.c:75:8
    #2 isc_taskmgr_create lib/isc/task.c:1419:3
    #3 create_managers bin/named/./main.c:902:11
    #4 setup bin/named/./main.c:1235:11
    #5 main bin/named/./main.c:1515:2

SUMMARY: ThreadSanitizer: lock-order-inversion (potential deadlock) in pthread_rwlock_wrlock