Why volatile and MemoryBarrier do not prevent operations reordering?

Question

If I understand meaning of volatile and MemoryBarrier correctly than the program below has never to be able to show any result.

It catches reordering of write operations

Answer 1

You aren't cleaning the variables between tests, so (for all but the first) initially a is 2 and b is 20 - before Write has done anything.

Check can get that initial value of a (so tempA is 2), and then Write can get in, get as far as changing b to 10.

Now Check reads the b (so tempB is 10).

Et voila. No re-order necessary to repro.

Reset a and b to 0 between runs and I expect it will go away.

edit: confirmed; "as is" I get the issue almost immediately (<2000 iterations); but by adding:

while (continueTrying)
{
    a = b = 0; // reset <======= added this

it then loops for any amount of time without any issue.

Or as a flow:

Write                   A=  B=        Check

(except first run)      2   20
                                      int tempA = a;
a = 1;                  1   20
Thread.MemoryBarrier();
b = 10;                 1   10
                                      int tempB = b;