CS 3733 Operating Systems

Read Sections 8.1 PUP

Last time we discussed two software solutions to the critical section problem that didn't work. In this lecture we will talk about solutions that do work.

• Example 3: (Algorithm 3 on page 161 of SG.)

       int flag[2] = {0, 0};    /* shared variable */
       int turn = 0;            /* shared variable */
  
  PROCESS 0:                        PROCESS 1:
  while(1) {                        while(1){
      flag[0] = 1;                      flag[1] = 1;
      turn = 1;                         turn = 0;
      while (flag[1] && turn = 1);      while (flag[0] && turn == 0);
                      
      flag[0] = 0;                      flag[1] = 0;
                     
  }                                 }
  

• Real implementations:
  • Low level synchronization is accomplished by hardware instructions such as:
    • test-and-set (Example 8.5 of PUP)
    • swap
  • Synchronization primitives such as semaphores are implemented by low-level synchronization.
  • Examples:
    • Using test-and-set to protect a critical section (Example 8.6 of PUP)
    • Using swap to protect a critical section (Example 8.7 of PUP)

Skill: Understand software and hardware locking mechanisms.