Supporting Multiple Threads

• Problems arise when two processes, or threads, try to access the same memory at the same time.
• Assume we have two programs running which both access the same memory location to keep a total for a bank account.
  • They have a section that looks like this

  • while (lock != 0) 
       no-op 
    lock = 0
    balance = balance + transaction
    lock = 1 
    	

  • Will this assure that two processes running the same code will not be able to access balance?
  • What does the while loop look like?

  • 0 locktop:
    1          lw	$t1, lock($zero)
    2 	   bne	$t1, $zero locktop
    3          sw	$t2, lock($zero)    # assume $t2 has a non zero value
    4	   lw	$t3, balance($zero)
    5	   add	$t3, $t4, $t4       # assume $t4 has the transaction
    6	   sw	$t3, balance($zero)
    7	   sw   $zero, lock($zero)
    	

  • What happens inf p1 is at line 2, the value of lock is 0 but the process gets interrupted?
  • Then what if p2, comes along, executes through line 4 and is interrupted?
  • Could this occur in other orders?
  • Lines 4-6 are known as a critical section.
  • The condition we have described requires mutual exclusion, or the ability to make sure that only one process can be in the critical section at any time.
  • This requires special hardware to accomplish this.
  • In a sense, it requires a load and and write to memory in a single, uninterruptable, or atomic, operation.
  • ll $t1,lock($t2) , load linked
  • R[$t1] = M[R[$t2]+lock]
  • This doesn't look special, but it is.
  • sc $t1, lock($t2)
  • M[R[$t0]+ lock] = R[$t1] , R[$t1] = 1,if the previous instruction was ll on this processor, and no other processor accessed this memory location.
  • Otherwise: M[$t1] = 0
  • The code becomes

    try:
    1	ll	$t1,lock($t2)
    2	sc	$t1, lock($t2)
    3	beq	$t1, $zero, try
    4       lw	$t3, balance($zero)
    5       add	$t3, $t4, $t4       # assume $t4 has the transaction
    6       sw	$t3, balance($zero)
    7       sw	$zero, lock($zero)