Consider how to implement a mutex lock using an atomic hardware instruction. Ass

Question

Consider how to implement a mutex lock using an atomic hardware instruction. Assume that the following structure defining the mutex lock is available: typedef struct {int available;} lock; (available == 0) indicates that the lock is available, and a value of 1 indicates that the lock is unavailable. Using this struct, illustrate how the following functions can be implemented using the test_and_set () and compare_and_swap() instructions: void acquire(lock *mutex) void release(lock *mutex) Be sure to include any initialization that may be necessary.

Explanation / Answer

Consider how to implement a mutex lock using an atomic hardware instruction. Assume that the following structure defining the mutex lock is available:
typedef struct {
int available;
} lock;
(available == 0) indicates that the lock is available, and a value of 1 indicates that the lock is unavailable. Using this struct, illustrate
how the following functions can be implemented using the test and set() and compare and swap() instructions:
• void acquire(lock *mutex)
• void release(lock *mutex)
Be sure to include any initialization that may be necessary.

// program for implementing the above problem

Test and Set:
-------------
typedef struct
{
int available;
}lock;

void init(lock *mutex)
   {
   // available==0 -> lock is available, available==1 -> lock unavailable
   mutex->available=0;
   }

int test_And_Set(int *target)
   {
int rv = *target;
*target = true;
return rv
   }

void acquire(lock *mutex)
   {
   while(test_and_set(&mutex->available,1)==1);
}
void release(lock *mutex)
   {
   mutex->available=0;
   }   

Compare and Swap:   
-----------------
int compare_and_Swap(int *ptr,int expected,int new)
   {
int actual = *ptr;
if(actual == expected)
   *ptr = new;
return actual;
   }

void acquire(lock *mutex)
   {
   while(compare_and_swap(&mutex->available,0,1)==1);
   }
void release(lock *mutex)
   {
        mutex->available=0;
   }

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