LU-9964 fixed the crashes but made unlocks synchronous, as the result group unlock may take dozens of seconds. Precisely, the test is N ranks opening (+O_CREAT) the same file, taking a group lock, writing 1MB segments for a total of 128GB (each rank writes 128GB/rank count) at a stride of (1MB*rank count), fysnc() and group unlock. Timings are similar regardless of MPI barriers after each phase or no barriers.

The ticket is to make group unlock asynchronous.