Question

Following is the hardware TestAndSet solution for critical-section problem. Can this solution satisfy bounded waiting? Explain your answer.

Shared boolean variable: lock (initialized to false)

Solution:

while (true) {

while(TestAndSet(&lock))

; /*do nothing

//critical section

lock = FALSE;

//remainder section

}

Answer 1

TestAndSet is a hardware solution to the synchronization problem. In TestAndSet, we have a shared lock variable which can take either of the two values, 0 or 1.

0 Unlock
1 Lock

Before entering into the critical section, a process inquires about the lock. If it is locked, it keeps on waiting until it becomes free and if it is not locked, it takes the lock and executes the critical section.

In TestAndSet, Mutual exclusion and bounded waiting can be preserved. For Example:-

you have n processes, process i is currently executing, when it enters the critical section, it sets its waiting to false.

Now, what happens when process i has finished ? Process i will look for an index j (process j) that is waiting to run in the critical section. In other words, we are looking for a process that is waiting to enter the critical section that is different from the current process that ran in a critical section

Process 1    Process 2

wants to set target to TRUE

target is changed to TRUE

the result comes back to FALSE so no busy waiting

wants to set target to TRUE

it receives the result TRUE

busy waits as long as Process 1 is in the critical section

leaves critical section

sets target to FALSE