No more milk - some solutions

• Never more than one person buys milk, and someone buys if needed
• Basic Idea (Sol 1) :
  Leave a note (kind of like a "lock")
  Remove note (kind of like an "unlock")
  if note present - don't buy (wait)
• Use atomic load and store as building blocks

if (noMilk) {
   if (noNote) {
	leave Note;
	buy milk;
	remove note; } }

• Process (thread) can get context switched after checking milk and leaving note, but before buying milk
• Bad solution - occasional failure - hard to detect

Solution 2

Process A		

leave noteA	
if (noNoteB) { 
 if (noMilk)
  buy milk }
remove noteA

ProcessB

leave noteB
if (noNoteA) {
 if (noMilk)
  buy milk }
remove noteB

• Context switch at the wrong time - possible for neither process to buy milk - as each thinks the other is buying milk
• Starvation : processes wait forever

Solution 3

Process A

leave note A
while (note B) // Pos: 1
  do nothing ; if
 (noMilk)
   buy milk ;
 remove noteA

Process B

leave note B
if (noNoteA) { // Pos: 2
  if (noMilk)
   buy milk }
  remove note B

• At Pos1: if noNote A, safe for B to buy (means A has not started yet). If note, A is either buying, or waiting for B to quit, so ok for B to quit
• At Pos2: if noNote B, safe to buy. If note B, don't know. A hangs around. Either: if B buys, done. If B doesn't buy, A will.
• Too complicated - must be simpler way