blk: clean up plug

Current code looks like inner plug gets flushed with a
blk_finish_plug(). Actually it's a nop. All requests/callbacks are added
to current->plug, while only outmost plug is assigned to current->plug.
So inner plug always has empty request/callback list, which makes
blk_flush_plug_list() a nop. This tries to make the code more clear.
Signed-off-by: Shaohua Li <shli@fb.com>
Reviewed-by: Jeff Moyer <jmoyer@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <axboe@fb.com>

block/blk-core.c

@@ -3032,21 +3032,20 @@ void blk_start_plug(struct blk_plug *plug)
 {
 	struct task_struct *tsk = current;
 
+	/*
+	 * If this is a nested plug, don't actually assign it.
+	 */
+	if (tsk->plug)
+		return;
+
 	INIT_LIST_HEAD(&plug->list);
 	INIT_LIST_HEAD(&plug->mq_list);
 	INIT_LIST_HEAD(&plug->cb_list);
-
-	/*
-	 * If this is a nested plug, don't actually assign it. It will be
-	 * flushed on its own.
-	 */
-	if (!tsk->plug) {
 	/*
 	 * Store ordering should not be needed here, since a potential
 	 * preempt will imply a full memory barrier
 	 */
 	tsk->plug = plug;
-	}
 }
 EXPORT_SYMBOL(blk_start_plug);
 
@@ -3193,9 +3192,10 @@ void blk_flush_plug_list(struct blk_plug *plug, bool from_schedule)
 
 void blk_finish_plug(struct blk_plug *plug)
 {
+	if (plug != current->plug)
+		return;
 	blk_flush_plug_list(plug, false);
 
-	if (plug == current->plug)
 	current->plug = NULL;
 }
 EXPORT_SYMBOL(blk_finish_plug);