Sorry to bother you, now I know that there is no problem here.
The model for reading and writing of PGXACT::xid and ShmemVariableCache->latestCompletedXid can be simplified as
follows:
backend A backend B backend C
wlock(XidGenLock); wlock(XidGenLock); rlock(ProcArrayLock);
write APgXact->xid; write BPgXact->xid; read latestCompletedXid;
unlock(XidGenLock); unlock(XidGenLock); read APgXact->xid;
... read BPgXact->xid;
wlock(ProcArrayLock); unlock(ProcArrayLock);
write latestCompletedXid;
unlock(ProcArrayLock);
My previous problem was that C might not be able to see the value of APgXact->xid written by A because there was no
obviousacquire-release operation during this. But now I find that there are already some acquire-release operations
here.Because of the `unlock(XidGenLock)` in A and `wlock(XidGenLock)` in B and the rules introduced in [Inter-thread
happens-before](https://en.cppreference.com/w/cpp/atomic/memory_order),we can know that the `write APgXact->xid` in A
inter-threadhappens before `write BPgXact->xid` in B. And `write BPgXact->xid` is sequenced before `write
latestCompletedXid`in B according to rules introduced in [Sequenced-before
rules](https://en.cppreference.com/w/cpp/language/eval_order).And similarly `write latestCompletedXid` in B
inter-threadhappens before `read latestCompletedXid` in C. So the `write APgXact->xid` in A inter-thread happens before
`readAPgXact->xid` in C. So C can see the value of APgXact->xid written by A.
