1. Thread Synchronization
Section 5
Thread Synchronization
February 17th, 2016
Taught by Joshua Don

2. Project Tips Keep the spec and your design doc handy
Make a plan for breaking up the work
Pair programming, group programming
Work backwards from the due date to decide when you want to complete each task by
You have the source code for all the tests
Make a separate branch (don’t always push to master!)
git checkout –b dev
git checkout master
git branch -v
git merge dev

3. Project Tips Cont. Task difficulty
Really important files for proj1 that you should understand completely:
timer.c
synch.c/h
thread.c/h
Easy
Hard
Task difficulty
Efficient alarm clock
Priority scheduling + priority donation
MLFQS
Staff solution: 500 lines
Personal: 363 lines

4. Atomic operations
Atomic operation: An operation that either executes completely or not at all. Cannot be interrupted in the middle of performing its function.
Is i++ atomic?
load i
increment value
store i
Race condition

5. Atomic operations i=0; i++; Thread 1 Thread 2 load i increment value
store i
Race condition

6. Atomic operations i=0; i++; Thread 2 load i Thread 1 increment value
store i
Thread 1
Race condition

7. Atomic operations i=0; i++; Thread 2 load i increment value store i
Race condition
Thread 1

8. Atomic operations i=0; i++; load i Thread 2 increment value store i
Race condition
Thread 1

9. Atomic operations i=0; i++; load i increment value store i Thread 1
Race condition
Thread 1
Thread 2

10. Atomic operations i=0; i++; load i increment value store i Thread 2
Race condition
Thread 2
Thread 1

11. Atomic operations i=0; i++; load i increment value store i Thread 1
Race condition
Thread 1
Thread 2

12. Atomic operations i=1 i=0; i++; load i increment value store i
Race condition
Thread 1
Thread 2
i=1

13. Test and Set Lock acquire Lock release
Test_and_set: An atomic operation that can be used to build a lock.
int test_and_set(int *value) {
int result = *value;
*value = 1;
return result; }
Simple lock using test_and_set
Busy wait
int value = 0;
...
while(test_and_set(&value));
// critical section
value = 0;
Lock acquire
Lock release

14. Critical Sections
Critical Section: A protected region of code. It requires mutual exclusion of access.
i=0
lock_acquire(my_lock);
i++;
lock_release(my_lock);

15. Demo