From f6c44e1b55c1bc98fca275f2040912cd89b8196a Mon Sep 17 00:00:00 2001
From: Alexander Korotkov <akorotkov@postgresql.org>
Date: Thu, 13 Dec 2018 06:12:25 +0300
Subject: [PATCH] Prevent deadlock in ginRedoDeletePage()

On standby ginRedoDeletePage() can work concurrently with read-only queries.
Those queries can traverse posting tree in two ways.
1) Using rightlinks by ginStepRight(), which locks the next page before
   unlocking its left sibling.
2) Using downlinks by ginFindLeafPage(), which locks at most one page at time.

Original lock order was: page, parent, left sibling.  That lock order can
deadlock with ginStepRight().  In order to prevent deadlock this commit changes
lock order to: left sibling, page, parent.  Note, that position of parent in
locking order seems insignificant, because we only lock one page at time while
traversing downlinks.

Reported-by: Chen Huajun
Diagnosed-by: Chen Huajun, Peter Geoghegan, Andrey Borodin
Discussion: https://postgr.es/m/31a702a.14dd.166c1366ac1.Coremail.chjischj%40163.com
Author: Alexander Korotkov
Backpatch-through: 9.4
---
 src/backend/access/gin/ginxlog.c | 22 +++++++++++++---------
 1 file changed, 13 insertions(+), 9 deletions(-)

diff --git a/src/backend/access/gin/ginxlog.c b/src/backend/access/gin/ginxlog.c
index 57ed81a8366..53b165326c5 100644
--- a/src/backend/access/gin/ginxlog.c
+++ b/src/backend/access/gin/ginxlog.c
@@ -511,6 +511,19 @@ ginRedoDeletePage(XLogReaderState *record)
 	Buffer		lbuffer;
 	Page		page;
 
+	/*
+	 * Lock left page first in order to prevent possible deadlock with
+	 * ginStepRight().
+	 */
+	if (XLogReadBufferForRedo(record, 2, &lbuffer) == BLK_NEEDS_REDO)
+	{
+		page = BufferGetPage(lbuffer);
+		Assert(GinPageIsData(page));
+		GinPageGetOpaque(page)->rightlink = data->rightLink;
+		PageSetLSN(page, lsn);
+		MarkBufferDirty(lbuffer);
+	}
+
 	if (XLogReadBufferForRedo(record, 0, &dbuffer) == BLK_NEEDS_REDO)
 	{
 		page = BufferGetPage(dbuffer);
@@ -530,15 +543,6 @@ ginRedoDeletePage(XLogReaderState *record)
 		MarkBufferDirty(pbuffer);
 	}
 
-	if (XLogReadBufferForRedo(record, 2, &lbuffer) == BLK_NEEDS_REDO)
-	{
-		page = BufferGetPage(lbuffer);
-		Assert(GinPageIsData(page));
-		GinPageGetOpaque(page)->rightlink = data->rightLink;
-		PageSetLSN(page, lsn);
-		MarkBufferDirty(lbuffer);
-	}
-
 	if (BufferIsValid(lbuffer))
 		UnlockReleaseBuffer(lbuffer);
 	if (BufferIsValid(pbuffer))