For any of the times that softirq's execute, a function do_softirq() is called and the following code executes.

unsigned int pending;

pending = local_softirq_pending();

if ( pending ) {
  struct softirq_action *h;

  /* clear the pending mask */
  set_softirq_pending(0);

  h = softirq_vec;
  do {
    if ( pending & 1 ) {
      h->action(h);
    }
    h++;
    pending >>= 1;
  } while( pending );
 }

• interrupts are disabled around geting a copying of the pending mask and clearing the mask!
• loop terminates ok because pending is 32 bits, there are 32 entries in the softirq_vec, and 32 (logical) right shifts will make pending be 0.