From: Noah Misch <noah@leadboat.com>
Date: Sat, 19 Oct 2019 03:21:10 +0000 (-0700)
Subject: Use standard compare_exchange loop style in ProcArrayGroupClearXid().
X-Git-Url: https://granicus.if.org/sourcecode?a=commitdiff_plain;h=48cc59ed24f95fa171b12ba1b461e6dc72d62b2b;p=postgresql

Use standard compare_exchange loop style in ProcArrayGroupClearXid().

Besides style, this might improve performance in the contended case.

Reviewed by Amit Kapila.

Discussion: https://postgr.es/m/20191015035348.GA4166224@rfd.leadboat.com
---

diff --git a/src/backend/storage/ipc/procarray.c b/src/backend/storage/ipc/procarray.c
index 8abcfdf841..3da53074b1 100644
--- a/src/backend/storage/ipc/procarray.c
+++ b/src/backend/storage/ipc/procarray.c
@@ -493,9 +493,9 @@ ProcArrayGroupClearXid(PGPROC *proc, TransactionId latestXid)
 	/* 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,