Race condition; sleep() and wakeup()

Race condition

A race condition is a situation in which two or more processes read and write shared data and the final result depends on who runs exactly when. Example: in a print spooler, two processes read the same free slot in = 7 and both write their file name in slot 7; one job is lost. Race conditions are avoided by mutual exclusion.

Sleep() and Wakeup() versus busy waiting

Busy-waiting solutions (Peterson, TSL, strict alternation) keep a process in a loop testing a variable until it may enter. Sleep and wakeup are system calls: sleep() blocks the caller (moves it to the waiting state) and wakeup(p) moves process p back to the ready state.

Producer-consumer example:

void producer() { while (TRUE) { item = produce();
    if (count == N) sleep();            /* buffer full */
    insert(item); count++;
    if (count == 1) wakeup(consumer); } }
void consumer() { while (TRUE) {
    if (count == 0) sleep();            /* buffer empty */
    item = remove(); count--;
    if (count == N-1) wakeup(producer);
    consume(item); } }

The producer sleeps when the buffer is full instead of spinning, and the consumer wakes it after removing an item. Caution: this plain version still has a race on count (a wakeup sent just before the other process sleeps is lost, and both may sleep forever); semaphores fix this by making the test and sleep atomic.