TSL instruction reference

Question

I want to use "TSL" instruction in assembly , but it has no reference for understand .in some articles this instruction is introduced for mutual exclusion problem but it has no reference or complete example to understand completely.

Answer 1

TSL (Test and Set Lock) is an operation that frequently comes up when dealing with mutual exclusion problems in general, but that doesn't mean such an instruction actually exists on whatever architecture you are using; or, even if it does exist, that it's called TSL.

On x86, for example, you can use the XCHG instruction to perform a TSL.

Answer 2

I think @Jester and @Seva Alekseyev have already done a great job answering this question but here is a simple implementation in C with inline Assembly by using pthreads on a x86 Ubuntu machine.

In the following example, there are two long-running threads. Both threads have a critical and a non-critical section, critical_region() and noncritical_region respectively. They both have to call enter_region() in order to get the lock. Once a thread gets the lock, it can start running its critical section. The other thread is blocked until the thread with the lock will call leave_region().

#include <stdio.h>
#include <unistd.h>
#include <pthread.h>
#include <stdlib.h>


void enter_region() {
    asm(
        ".data\n\t"
        "lock:\n\t"
        ".byte 0\n\t"
        ".text\n\t"

        "_enter_region:\n\t"
        "movb $1, %al\n\t" /* move 1 to AL */
        "xchgb (lock),%al\n\t" 
        "cmp $0, %al\n\t"
        "jne _enter_region\n\t"

    );
}

void leave_region() {
    asm("movb $0, (lock)");
}


void critical_region() {

}

void noncritical_region() {

}

static void* f1(void* p) {
    while(1) {
        puts("wait for f2");
        enter_region();
        printf("f1 can start its critical section\n");
        critical_region();
        leave_region();
        noncritical_region();
    }
    return NULL;
}

static void* f2(void* p) {
    while(1) {
        puts("wait for f1");
        enter_region();
        printf("f2 can start its critical section\n");
        critical_region();
        leave_region();

        /* if you call sleep, you can see that the non-critical section of this thread won't
        *  block the other thread from running its critical section as many times as it wants
        */
        // sleep(1);
        noncritical_region();
    }
    return NULL;
}

int main() {
    int rc;

    pthread_t t1, t2;

    rc = pthread_create(&t1, NULL, f1, NULL);
    if(rc != 0) {
        fprintf(stderr, "pthread f1 failed\n");
        return EXIT_FAILURE;
    }

    rc = pthread_create(&t2, NULL, f2, NULL);
    if(rc != 0) {
        fprintf(stderr, "pthread f2 failed\n");
        return EXIT_FAILURE;
    }

    pthread_join(t1, NULL);
    pthread_join(t2, NULL);

    puts("All threads finished.");
    return 0;
}

Answer 3

XCNG is not conditional, it just exchanges a register with a memory location. What you want is CMPXCHG with the LOCK prefix. The latter will make it atomic on multicore machines.

Also, you can implement atomic compare-and-set with LOCK XADD, but that'll take a loop.

Ref: http://en.wikipedia.org/wiki/Compare-and-swap