Assume we have two concurrent threads/processes, each with
simple straightline code and sharing a global variable
x
.
Assume that each individual statement is atomic.
1 thread0:
2 {
3 x=0
4 x=x+1
5 }
6 thread1:
7 {
8 x=0
9 x=x+1
10 x=x+2
11 }
Without using SPIN,
give one trace illustrating each of the possible final values
of x
.
There are three possible final values of
x
:
1, 3, 4.

Answer the previous exercise, except with different atomicity. Assume that each variable lookup, variable assignment, and addition are atomic, as in a typical machine language. Specifically, use the following code:
1 thread0:
2 {
3 x = 0
4 r0 = x
5 r0 += 1
6 x = r0
7 }
8 thread1:
9 {
10 x = 0
11 r1 = x
12 r1 += 1
13 x = r1
14 r1 = x
15 r1 += 2
16 x = r1
17 }
The values 1, 3, and 4 are possible as before. It should be clear that values greater than 4 are impossible, since at most a total of 4 can ever be added. Similarly, values less than 1 are impossible, since at least 1 is added after the last variable lookup. So, we're left with the question of whether 2 is a possible result. It is, and one possible trace is given.