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C Z6.0+pooncerelease+poacquirerelease+fencembonceonce
(*
* Result: Sometimes
*
* This litmus test shows that a release-acquire chain, while sufficient
* when there is but one non-reads-from (AKA non-rf) link, does not suffice
* if there is more than one. Of the three processes, only P1() reads from
* P0's write, which means that there are two non-rf links: P1() to P2()
* is a write-to-write link (AKA a "coherence" or just "co" link) and P2()
* to P0() is a read-to-write link (AKA a "from-reads" or just "fr" link).
* When there are two or more non-rf links, you typically will need one
* full barrier for each non-rf link. (Exceptions include some cases
* involving locking.)
*)
{}
P0(int *x, int *y)
{
WRITE_ONCE(*x, 1);
smp_store_release(y, 1);
}
P1(int *y, int *z)
{
int r0;
r0 = smp_load_acquire(y);
smp_store_release(z, 1);
}
P2(int *x, int *z)
{
int r1;
WRITE_ONCE(*z, 2);
smp_mb();
r1 = READ_ONCE(*x);
}
exists (1:r0=1 /\ z=2 /\ 2:r1=0)
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