Implementing a Mutex Lock in C
- by Adam
I'm trying to make a really mutex in C and for some reason I'm getting cases where two threads are getting the lock at the same time, which shouldn't be possible. Any ideas why it's not working?
void mutexLock(mutex_t *mutexlock, pid_t owner)
{
int failure;
while(mutexlock->mx_state == 0 || failure || mutexlock->mx_owner != owner)
{
failure = 1;
if (mutexlock->mx_state == 0)
{
asm(
"test:"
"movl $0x01,%%eax\n\t" // move 1 to eax
"xchg %%eax,%0\n\t" // try to set the lock bit
"mov %%eax,%1\n\t" // export our result to a test var
"test %%eax,%%eax\n\t"
"jnz test\n\t"
:"=r"(mutexlock->mx_state),"=r"(failure)
:"r"(mutexlock->mx_state)
:"%eax"
);
}
if (failure == 0)
{
mutexlock->mx_owner = owner; //test to see if we got the lock bit
}
}
}
