WiredTigerKVEngine::beginNonBlockingBackup acquires _oplogPinnedByBackupMutex before acquiring _wtBackup.wtBackupDupCursorMutex (see here).

But WiredTigerKVEngine::endNonBlockingBackup acquires these mutexes in the inverted order: https://github.com/10gen/mongo/blob/16f711ffff54f0cf019efda3dc7796c39f7fe615/src/mongo/db/storage/wiredtiger/wiredtiger_kv_engine.cpp#L1365-L1369.

Both of these functions acquire _wtBackup.wtBackupCursorMutex first/before either of these mutexes, so there isn't a real deadlock today. But it makes the code hard to reason about, bug-prone in the future, and requires a TSAN suppression to run without these locks having a clear ordering.