block: fix request sorting at unplug

koct9i · Jens Axboe · commit f83e826181f7 · 2011-04-05T23:52:49.000+02:00
Comparison function for list_sort() must be anticommutative,
otherwise it is not sorting in ordinary meaning.

But fortunately list_sort() always check ((*cmp)(priv, a, b) &lt;= 0)
it not distinguish negative and zero, so comparison function can
implement only less-or-equal instead of full three-way comparison.

Signed-off-by: Konstantin Khlebnikov &lt;khlebnikov@openvz.org&gt;
Signed-off-by: Jens Axboe &lt;jaxboe@fusionio.com&gt;
diff --git a/block/blk-core.c b/block/blk-core.c
@@ -2665,7 +2665,7 @@ static int plug_rq_cmp(void *priv, struct list_head *a, struct list_head *b)
 	struct request *rqa = container_of(a, struct request, queuelist);
 	struct request *rqb = container_of(b, struct request, queuelist);
 
-	return !(rqa->q == rqb->q);
+	return !(rqa->q <= rqb->q);
 }
 
 static void flush_plug_list(struct blk_plug *plug)

Original file line number	Diff line number	Diff line change
`@@ -2665,7 +2665,7 @@ static int plug_rq_cmp(void priv, struct list_head a, struct list_head *b)`
`2665`	`2665`	`struct request *rqa = container_of(a, struct request, queuelist);`
`2666`	`2666`	`struct request *rqb = container_of(b, struct request, queuelist);`
`2667`	`2667`
`2668`		`- return !(rqa->q == rqb->q);`
	`2668`	`+ return !(rqa->q <= rqb->q);`
`2669`	`2669`	`}`
`2670`	`2670`
`2671`	`2671`	`static void flush_plug_list(struct blk_plug *plug)`