The following C code was our first attempt to make a lock using a flag to indicate if the lock was acquired.
typedef struct __lock_t { int flag; } lock_t;
void init(lock_t *mutex) {
mutex-flag = 0;
}
void lock(lock_t *mutex) {
while (mutex-flag == 1) // TEST the flag
; // spin-wait (do nothing)
mutex-flag = 1; // now SET it !
}
void unlock(lock_t *mutex) {
mutex-flag = 0;
}

How will this code perform in terms of correctness and efficiency?

Question

The following C code was our first attempt to make a lock using a flag to indicate if the lock was acquired. 
 typedef struct __lock_t { int flag; } lock_t;
void init(lock_t *mutex) {
   mutex->flag = 0;
}
void lock(lock_t *mutex) {
   while (mutex->flag == 1)  // TEST the flag
      ;  // spin-wait (do nothing)
   mutex->flag = 1;  // now SET it !
}
void unlock(lock_t *mutex) {
   mutex->flag = 0;
}
 
 How will this code perform in terms of correctness and efficiency?

Accepted Answer

The code is not correct as it does not always provide mutual exclusion, and is very inefficient.

Answer

The code is correct because it provides mutual exclusion, but it is inefficient.

Answer

The code is correct because it provides mutual exclusion, and it is efficient.

Answer

The code is not correct as it does not always provide mutual exclusion, but when it does work it is efficient.

Crowdly

The following C code was our first attempt to make a lock using a flag to indica...

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