1. Lecture 12: Peterson’s Solution and Hardware Support

2. Review: Mutual Exclusion
Critical region
Disabling interrupts
Strict alternation

3. In this lecture Mutual exclusion solutions Peterson’s solution
Hardware support

4. General structure
enter_region(); critical_region(); leave_region();

5. Peterson’s Solution (1981)

6. Hardware Support: TSL
New Instruction: TSL R, Lock TSL = “Test and Set Lock” R = register, Lock = memory location
Atomically (atomic = nothing can interfere) - Read Lock into R - Store a non-zero value into Lock

7. Mutual Exclusion using TSL
/* set lock to non-zero, proceed if it was 0 earlier */
enter_region: TSL Reg, Lock /* set Lock to 1, copy old value of Lock into Reg*/
If (Reg != 0) then /* not the first to set to zero */
Jump enter_region /* try again */
return
leave_region:
Lock = 0

8. Alternative to TSL: XCHG
/* set lock to non-zero, proceed if it was 0 earlier */
enter_region:
Reg = 1 XCHG Reg, Lock /* swap contents of Reg and Lock */
If (Reg != 0) then /* not the first to set to zero */
Jump enter_region /* try again */
return
leave_region:
Lock = 0

9. Problem
Busy waiting
Possible solution: Yield to another thread if unable to lock first time

10. Eliminate Busy Waiting
/* set lock to non-zero, proceed if it was 0 earlier */
enter_region: TSL Reg, Lock /* set Lock to 1, copy old value of Lock into Reg*/
If (Reg != 0) then /* not the first to set to zero */ {
thread_yield() ; /* let somebody else run */
Jump enter_region /* try again */ }
return
leave_region:
Lock = 0

11. Summary of mutual exclusion solutions
Software solution
Disabling interrupts
Single processor only
Use in kernel mode
Strict alternation
Strict ordering
Busy waiting
Peterson’s solution
Hardware solution
TSL/XCHG
Works on multiprocessors

12. Avoiding Busy Waiting
If a process cannot enter critical region, the process calls sleep() to give up CPU.
A process wakes up another process using wakeup()

13. Review: Mutual Exclusion using TSL
/* set lock to non-zero, proceed if it was 0 earlier */
enter_region: TSL Reg, Lock /* set Lock to 1, copy old value of Lock into Reg*/
If (Reg != 0) then /* not the first to set to zero */
Jump enter_region /* try again */
return
leave_region:
Lock = 0

14. TSL without busy waiting
/* set lock to non-zero, proceed if it was 0 earlier */
enter_region: TSL Reg, Lock /* set Lock to 1, copy old value of Lock into Reg*/
If (Reg != 0) then /* not the first to set to zero */ {
sleep()
Jump enter_region /* try again */
return
leave_region:
Lock = 0
wakeup(process_id)