# Class 20: Mutual Exclusion

Posted: Thu 10 April 2014

PS4 Assessment due after your demo (by tomorrow at the latest)
Project Idea due by 11:59pm Thursday (tonight).

# Mutual Exclusion

Requirements:

1. Only one thread may be in the critical section at any time.
2. Each must eventually be able to enter its critical section.
3. Must be symmetrical (all run same program, cannot introduce static priority).

What's "wrong" with this solution?

```loop {
if turn == i:
critical_section;
turn = i + 1;
}
```

What's "wrong" with this solution?

```loop {
if not lock:
lock = true;
critical_section;
lock = false;
}
```

## Test and Set

Atomic instruction that: - returns current value of v - sets value of v to true

How can you implement mutual exclusion using test-and-set?

## ARM's Exclusion Instructions

LDREX dest location
dest = location
Sets monitor on location in Exclusive state

STREX success value location
Conditionally store into exclusive .
If permitted, = 1 and = .
If not, = 0 and value unchanged.

Context switch clears monitor (Open) state.

ARM Synchronization Primitives

```lock_mutex(lock):
try_again:
LDREX R2, [lock]
if R2 goto try_again
STREX R2, 1, [lock]
if not R2 goto try_again

unlock_mutex(lock):
STR [lock], 0
```

Why don't we need to use STREX in unlock?

How should this be different if we care about energy?

# Dijkstra's Solution

```   loop {
b[i] := false
L1:  if k != i
c[i] := true
if b[k]:
k := i
goto L1
else:
c[i] := false
for j in [1, ..., N]:
if j != i and not c[j]:
goto L1
critical section;
c[i] := true
b[i] := true
}
```

Why is this guaranteed to provide mutual exclusion?

How do we know threads will make progress?

Dijkstra's Cry for Generalization