1*4882a593SmuzhiyunC Z6.0+pooncerelease+poacquirerelease+fencembonceonce
2*4882a593Smuzhiyun
3*4882a593Smuzhiyun(*
4*4882a593Smuzhiyun * Result: Sometimes
5*4882a593Smuzhiyun *
6*4882a593Smuzhiyun * This litmus test shows that a release-acquire chain, while sufficient
7*4882a593Smuzhiyun * when there is but one non-reads-from (AKA non-rf) link, does not suffice
8*4882a593Smuzhiyun * if there is more than one.  Of the three processes, only P1() reads from
9*4882a593Smuzhiyun * P0's write, which means that there are two non-rf links: P1() to P2()
10*4882a593Smuzhiyun * is a write-to-write link (AKA a "coherence" or just "co" link) and P2()
11*4882a593Smuzhiyun * to P0() is a read-to-write link (AKA a "from-reads" or just "fr" link).
12*4882a593Smuzhiyun * When there are two or more non-rf links, you typically will need one
13*4882a593Smuzhiyun * full barrier for each non-rf link.  (Exceptions include some cases
14*4882a593Smuzhiyun * involving locking.)
15*4882a593Smuzhiyun *)
16*4882a593Smuzhiyun
17*4882a593Smuzhiyun{}
18*4882a593Smuzhiyun
19*4882a593SmuzhiyunP0(int *x, int *y)
20*4882a593Smuzhiyun{
21*4882a593Smuzhiyun	WRITE_ONCE(*x, 1);
22*4882a593Smuzhiyun	smp_store_release(y, 1);
23*4882a593Smuzhiyun}
24*4882a593Smuzhiyun
25*4882a593SmuzhiyunP1(int *y, int *z)
26*4882a593Smuzhiyun{
27*4882a593Smuzhiyun	int r0;
28*4882a593Smuzhiyun
29*4882a593Smuzhiyun	r0 = smp_load_acquire(y);
30*4882a593Smuzhiyun	smp_store_release(z, 1);
31*4882a593Smuzhiyun}
32*4882a593Smuzhiyun
33*4882a593SmuzhiyunP2(int *x, int *z)
34*4882a593Smuzhiyun{
35*4882a593Smuzhiyun	int r1;
36*4882a593Smuzhiyun
37*4882a593Smuzhiyun	WRITE_ONCE(*z, 2);
38*4882a593Smuzhiyun	smp_mb();
39*4882a593Smuzhiyun	r1 = READ_ONCE(*x);
40*4882a593Smuzhiyun}
41*4882a593Smuzhiyun
42*4882a593Smuzhiyunexists (1:r0=1 /\ z=2 /\ 2:r1=0)
43