compare-exchange-style-v1.patch

diff --git a/src/backend/storage/ipc/procarray.c b/src/backend/storage/ipc/procarray.c
index 8abcfdf..3da5307 100644
--- a/src/backend/storage/ipc/procarray.c
+++ b/src/backend/storage/ipc/procarray.c
@@ -490,15 +490,15 @@ ProcArrayGroupClearXid(PGPROC *proc, TransactionId latestXid)
 	/* We should definitely have an XID to clear. */
 	Assert(TransactionIdIsValid(allPgXact[proc->pgprocno].xid));
 
 	/* Add ourselves to the list of processes needing a group XID clear. */
 	proc->procArrayGroupMember = true;
 	proc->procArrayGroupMemberXid = latestXid;
+	nextidx = pg_atomic_read_u32(&procglobal->procArrayGroupFirst);
 	while (true)
 	{
-		nextidx = pg_atomic_read_u32(&procglobal->procArrayGroupFirst);
 		pg_atomic_write_u32(&proc->procArrayGroupNext, nextidx);
 
 		if (pg_atomic_compare_exchange_u32(&procglobal->procArrayGroupFirst,
 										   &nextidx,
 										   (uint32) proc->pgprocno))
 			break;