Do I use memory barrier the wrong way? - c++

Question

I simply implement Peterson Lock Algorithm, but it doesn't work rightly.
Here is the code:
#include <pthread.h>
typedef struct {
volatile bool flag[2];
volatile bool victim;
} peterson_lock_t;
void peterson_lock_init(peterson_lock_t &lock) {
lock.flag[0] = lock.flag[1] = false;
lock.victim = 0;
}
void peterson_lock(peterson_lock_t &lock, int id) {
lock.victim = id;
lock.flag[id] = true;
__asm__ __volatile__("" : : : "memory");
while (lock.flag[1 - id] == false && lock.victim != id);
}
void peterson_unlock(peterson_lock_t &lock, int id) {
lock.flag[id] = false;
}
What's wrong with this code?
main.cpp:
#include <stdio.h>
#include "peterson_lock.h"
peterson_lock_t lock;
int count = 0;
void *routine0(void *arg) {
int *cnt = (int *)arg;
for (int i = 0; i < *cnt; ++i) {
peterson_lock(lock, 0);
++count;
peterson_unlock(lock, 0);
}
return NULL;
}
void *routine1(void *arg) {
int *cnt = (int *)arg;
for (int i = 0; i < *cnt; ++i) {
peterson_lock(lock, 1);
++count;
peterson_unlock(lock, 1);
}
}
int main(int argc, char **argv) {
peterson_lock_init(lock);
pthread_t thread0, thread1;
int count0 = 10000;
int count1 = 20000;
pthread_create(&thread0, NULL, routine0, (void *)&count0);
pthread_create(&thread1, NULL, routine1, (void *)&count1);
pthread_join(thread0, NULL);
pthread_join(thread1, NULL);
printf("Expected: %d\n", (count0 + count1));
printf("Reality : %d\n", count);
return 0;
}
And the result is not right:
Expected: 30000
Reality : 24304
Expected: 30000
Reality : 24316
OS:
Linux ip-172-31-43-244 3.14.35-28.38.amzn1.x86_64 #1 SMP Wed Mar 11 22:50:37 UTC 2015 x86_64 x86_64 x86_64 GNU/Linux

Answer 1

While condition also needs to be improved :
while (lock.flag[1 - id] == true && lock.victim == id)
continue;
Be in busy wait until the other thread has lock and you are the victim.
Peterson Lock