commit b1ba9d8a5a647af3088a21597dc65af1631919ea
Author: Alexander Korotkov <akorotkov@postgresql.org>
Date:   Wed Dec 12 03:06:07 2018 +0300

    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 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.

diff --git a/src/backend/access/gin/ginxlog.c b/src/backend/access/gin/ginxlog.c
index 7701a2d6bfc..b626a219dec 100644
--- a/src/backend/access/gin/ginxlog.c
+++ b/src/backend/access/gin/ginxlog.c
@@ -513,6 +513,19 @@ 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);
@@ -532,15 +545,6 @@ 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))