Deadlock and race condition in mutex implementation - c

Question

I'm trying to implement a mutex in C using the atomic assembly instruction "bts" to atomically set a bit and return the original value.
However, when I run the following code, it occasionally deadlocks and often shows race conditions:
#include <stdio.h>
#include <stdlib.h>
#include <pthread.h>
typedef unsigned char mutex;
#define MUTEX_FREE 0
#define MUTEX_BUSY 1
// adapted from http://www.acm.uiuc.edu/sigops/roll_your_own/i386/atomic.html
mutex testAndSet(mutex *m) {
int result;
asm ("bts $0, %1; sbbl %0, %0"
:"=r" (result)
:"m" (*m)
:"memory");
return (result & 1);
}
void P(mutex *m) {
// Must use atomic testAndSet to avoid race conditions
while(testAndSet(m) == MUTEX_BUSY)
usleep(10);
}
void V(mutex *m) {
*m = MUTEX_FREE;
}
//////////////
// Test:
//////////////
const int NTHREADS = 100;
const int NINCS = 100;
int counter = 0;
mutex m = MUTEX_FREE;
void criticalSection() {
int i;
for(i=0;i<NINCS;i++) {
P(&m);
counter++;
V(&m);
}
}
int main() {
int i;
pthread_t threads[NTHREADS];
for(i=0; i<NTHREADS; i++) {
pthread_create(&threads[i], NULL, (void *) &criticalSection, NULL);
}
for(i=0; i<NTHREADS; i++) {
pthread_join(threads[i], NULL);
}
printf("got counter=%d, expected=%d\n", counter, NTHREADS*NINCS);
}
The code seems to work if I use the "xchgb" instruction instead of "bts" as follows:
mutex testAndSet(mutex *m) {
unsigned char result = MUTEX_BUSY;
asm ("xchgb %1, %0"
:"=m" (*m), "=r" (result)
:"1" (result)
:"memory");
return result;
}
Where is the race condition in the original code? Shouldn't the "bts" instruction be atomic, guaranteeing thread safety?
Furthermore, is my modified solution actually correct?
(I'm running OS X 10.8 and compiling with gcc.)

Answer 1

Try using the LOCK prefix to lock the memory bus:
asm ("lock bts $0, %1; ...");
The xchg instruction worked because that always asserts the LOCK# signal regardless of the presence or absence of the LOCK prefix.