The synchronization on org.hibernate.search.backend.impl.lucene.NRTWorkspaceImpl.setupNewReadersRequirements() is intended to be quick-returning and only needed to provide a consistent read to method refreshReaders(), but doesn't need to block on existing refreshReaders() invocations, especially as these might be needing to flush deletion buffers to disks.

So a refreshReaders() operation can be relatively slow, and this prevents a flush operation to return switfly.

This contention gets noticeable on high load exercise of parallel readers & writers on this backend.