Mutual Exclusion Problem

Question

Please take a look on the following pseudo-code:

boolean blocked[2];
int turn;
void P(int id) {
      while(true) {
             blocked[id] = true;
                 


        

Answer 1

Concurrency can not be implemented like this, especially in a multi-processor (or multi-core) environment: different cores/processors have different caches. Those caches may not be coherent. The pseudo-code below could execute in the order shown, with the results shown:

get blocked[0] -> false   // cpu 0
set blocked[0] = true     // cpu 1 (stored in CPU 1's L1 cache)
get blocked[0] -> false   // cpu 0 (retrieved from CPU 0's L1 cache)
get glocked[0] -> false   // cpu 2 (retrieved from main memory)

You need hardware knowledge to implement concurrency.