Lock and Unlock

• Use binary indicators to control access to a shared resource
• L = 1 (means resource is unavailable (locked)
  L = 0 (means resource is free (unlocked)
• Primitive operations on locks
  LOCK(L) ::= when L=0 do L=1 ;
  UNLOCK(L) :: L=0 ;
• Mutual exclusion can be obtained using locks

PROCESS P[i] ;
 begin
   LOCK(L) ;
   {critical section} 
   UNLOCK(L) ;
 end

Implementation of Locks

• The indivisible TESTandSET instruction
• single instruction that reads a variable, stores its value in an allocated area (critical section), and then sets the variable to a certain value
• Instruction once initiated will complete all operations without interruption
• It is difficult to use, understand and verify -and is also wasteful