Semaphores

Problems with the semaphore solution to the critical-section problem

  1. Subtle errors are easily made, and can lead to big problems. E.g. suppose a particular critical region needs access to data protected by two different semaphores S1 and S2. One process might code:

    Process 0 Process 1 
        P(S1); P(S2);
    critical region;  
    V(S1); V(S2);

        P(S2); P(S1);
    critical region;  
    V(S2); V(S1);

    which could lead to deadlock if the two processes both did their first P() before either did its second.