The operating system is allowed to terminate a process for various reasons. A process that does not cooperate is independent . Cooperating processes may be useful for several reasons, as detailed on pp. For example, consider a producer process and a consumer process. Threads are similar to processes in some ways.
Threads are different from processes in other ways.
B's threads can be executing a system call at the same time. An Introduction to Operating Systems , p. 587. Any sufficiently clever programmer can create user-level threads by hand. T12 enters the ready state. T21 enters the ready state.