Question

Suppose the following two processes, foo and bar, are executed concurrently and share the semaphore variables S and R (each initialized to 1) and the integer variable x (initialized to 0).

a. Can the concurrent execution of these two processes result in one or both being blocked forever? If yes, give an execution sequence in which one or both are blocked forever.
b. Can the concurrent execution of these two processes result in the indefinite postponement of one of them? If yes, give an execution sequence in which one is indefinitely postponed.

Answer 1

a. 

Yes. If foo( ) executes semWait(S) and then bar( ) executes semWait(R) both processes will then block when each executes its next instruction. Since each will then be waiting for a semSignal( ) call from the other, neither will ever resume execution.

 

 

 

b. 

No. If either process blocks on a semWait( ) call then either the other process will also block as described in (a) or the other process is executing in its critical section. In the latter case, when the running process leaves its critical section, it will execute a semSignal( ) call, which will awaken the blocked process.

No. If either process blocks on a semWait( ) call then either the other process will also block as described in (a) or the other process is executing in its critical section. In the latter case, when the running process leaves its critical section, it will execute a semSignal( ) call, which will awaken the blocked process.