A shared session, which can be shared across multiple threads. Closing a shared session has to ensure that (i) only one thread can successfully close a session (e.g. in a close vs. close race) and that (ii) no other thread is accessing the memory associated with this session while the segment is being closed. To ensure the former condition, a CAS is performed on the liveness bit. Ensuring the latter is trickier, and require a complex synchronization protocol (see jdk.internal.misc.ScopedMemoryAccess). Since it is the responsibility of the closing thread to make sure that no concurrent access is possible, checking the liveness bit upon access can be performed in plain mode, as in the confined case.