How to guarantee ordering of instruction in C

Question

My multithreaded code is as follows:

thread A

while(true) {
  foo();
  sleep();
}

thread B

...
bar_off();
bar_on();
...

foo() isn't safe to be executed when bar_off is called and until bar_on has completed.

I could rewrite my code as follows:

int volatile foo_is_safe=0;

thread A

while(true) {
  if (foo_is_safe) {
    foo();
  }
  sleep();
}

thread B

...
foo_is_safe=0;
bar_off();
bar_on();
foo_is_safe=1;
...

But I believe this won't work since the compiler is not required to preserve the ordering between setting foo_is_safe and calling the bar_* functions. I am not even sure the compiler will not simply ignore setting foo_is_safe to 0.

Am I correct? What's the proper way to make my code safe, i.e. enforce foo_is_safe is set before bar_off() and after bar_on()?