Correct nbtree page split lock coupling comment.

petergeoghegan · petergeoghegan · commit d129c07499db · 2020-08-09T12:01:15.000-07:00
There is no reason to distinguish between readers and writers here.
diff --git a/src/backend/access/nbtree/nbtinsert.c b/src/backend/access/nbtree/nbtinsert.c
@@ -1861,11 +1861,9 @@ _bt_split(Relation rel, BTScanInsert itup_key, Buffer buf, Buffer cbuf,
 	}
 
 	/*
-	 * We have to grab the right sibling (if any) and fix the prev pointer
-	 * there. We are guaranteed that this is deadlock-free since no other
-	 * writer will be holding a lock on that page and trying to move left, and
-	 * all readers release locks on a page before trying to fetch its
-	 * neighbors.
+	 * We have to grab the original right sibling (if any) and update its prev
+	 * link.  We are guaranteed that this is deadlock-free, since we couple
+	 * the locks in the standard order: left to right.
 	 */
 	if (!isrightmost)
 	{

Original file line number	Diff line number	Diff line change
`@@ -1861,11 +1861,9 @@ _bt_split(Relation rel, BTScanInsert itup_key, Buffer buf, Buffer cbuf,`
`1861`	`1861`	`}`
`1862`	`1862`
`1863`	`1863`	`/*`
`1864`		`- * We have to grab the right sibling (if any) and fix the prev pointer`
`1865`		`- * there. We are guaranteed that this is deadlock-free since no other`
`1866`		`- * writer will be holding a lock on that page and trying to move left, and`
`1867`		`- * all readers release locks on a page before trying to fetch its`
`1868`		`- * neighbors.`
	`1864`	`+ * We have to grab the original right sibling (if any) and update its prev`
	`1865`	`+ * link. We are guaranteed that this is deadlock-free, since we couple`
	`1866`	`+ * the locks in the standard order: left to right.`
`1869`	`1867`	`*/`
`1870`	`1868`	`if (!isrightmost)`
`1871`	`1869`	`{`