block: make every barrier action optional

In all barrier sequences, the barrier write itself was always assumed
to be issued and thus didn't have corresponding control flag.  This
patch adds QUEUE_ORDERED_DO_BAR and unify action mask handling in
start_ordered() such that any barrier action can be skipped.

This patch doesn't introduce any visible behavior changes.
Signed-off-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>

block/blk-barrier.c

@@ -158,19 +158,10 @@ static inline struct request *start_ordered(struct request_queue *q,
 	q->ordered = q->next_ordered;
 	q->ordseq |= QUEUE_ORDSEQ_STARTED;
 
-	/*
-	 * Prep proxy barrier request.
-	 */
+	/* stash away the original request */
 	elv_dequeue_request(q, rq);
 	q->orig_bar_rq = rq;
-	rq = &q->bar_rq;
-	blk_rq_init(q, rq);
-	if (bio_data_dir(q->orig_bar_rq->bio) == WRITE)
-		rq->cmd_flags |= REQ_RW;
-	if (q->ordered & QUEUE_ORDERED_DO_FUA)
-		rq->cmd_flags |= REQ_FUA;
-	init_request_from_bio(rq, q->orig_bar_rq->bio);
-	rq->end_io = bar_end_io;
+	rq = NULL;
 
 	/*
 	 * Queue ordered sequence.  As we stack them at the head, we
@@ -181,12 +172,28 @@ static inline struct request *start_ordered(struct request_queue *q,
 	 * there will be no data written between the pre and post flush.
 	 * Hence a single flush will suffice.
 	 */
-	if ((q->ordered & QUEUE_ORDERED_DO_POSTFLUSH) && !blk_empty_barrier(rq))
+	if ((q->ordered & QUEUE_ORDERED_DO_POSTFLUSH) &&
+	    !blk_empty_barrier(q->orig_bar_rq)) {
 		queue_flush(q, QUEUE_ORDERED_DO_POSTFLUSH);
-	else
+		rq = &q->post_flush_rq;
+	} else
 		q->ordseq |= QUEUE_ORDSEQ_POSTFLUSH;
 
+	if (q->ordered & QUEUE_ORDERED_DO_BAR) {
+		rq = &q->bar_rq;
+
+		/* initialize proxy request and queue it */
+		blk_rq_init(q, rq);
+		if (bio_data_dir(q->orig_bar_rq->bio) == WRITE)
+			rq->cmd_flags |= REQ_RW;
+		if (q->ordered & QUEUE_ORDERED_DO_FUA)
+			rq->cmd_flags |= REQ_FUA;
+		init_request_from_bio(rq, q->orig_bar_rq->bio);
+		rq->end_io = bar_end_io;
+
 		elv_insert(q, rq, ELEVATOR_INSERT_FRONT);
+	} else
+		q->ordseq |= QUEUE_ORDSEQ_BAR;
 
 	if (q->ordered & QUEUE_ORDERED_DO_PREFLUSH) {
 		queue_flush(q, QUEUE_ORDERED_DO_PREFLUSH);
@@ -194,10 +201,10 @@ static inline struct request *start_ordered(struct request_queue *q,
 	} else
 		q->ordseq |= QUEUE_ORDSEQ_PREFLUSH;
 
-	if ((q->ordered & QUEUE_ORDERED_BY_TAG) || q->in_flight == 0)
-		q->ordseq |= QUEUE_ORDSEQ_DRAIN;
-	else
+	if ((q->ordered & QUEUE_ORDERED_BY_DRAIN) && q->in_flight)
 		rq = NULL;
+	else
+		q->ordseq |= QUEUE_ORDSEQ_DRAIN;
 
 	return rq;
 }

include/linux/blkdev.h

@@ -526,12 +526,14 @@ enum {
 	QUEUE_ORDERED_BY_DRAIN		= 0x01,
 	QUEUE_ORDERED_BY_TAG		= 0x02,
 	QUEUE_ORDERED_DO_PREFLUSH	= 0x10,
+	QUEUE_ORDERED_DO_BAR		= 0x20,
 	QUEUE_ORDERED_DO_POSTFLUSH	= 0x40,
 	QUEUE_ORDERED_DO_FUA		= 0x80,
 
 	QUEUE_ORDERED_NONE		= 0x00,
 
-	QUEUE_ORDERED_DRAIN		= QUEUE_ORDERED_BY_DRAIN,
+	QUEUE_ORDERED_DRAIN		= QUEUE_ORDERED_BY_DRAIN |
+					  QUEUE_ORDERED_DO_BAR,
 	QUEUE_ORDERED_DRAIN_FLUSH	= QUEUE_ORDERED_DRAIN |
 					  QUEUE_ORDERED_DO_PREFLUSH |
 					  QUEUE_ORDERED_DO_POSTFLUSH,
@@ -539,7 +541,8 @@ enum {
 					  QUEUE_ORDERED_DO_PREFLUSH |
 					  QUEUE_ORDERED_DO_FUA,
 
-	QUEUE_ORDERED_TAG		= QUEUE_ORDERED_BY_TAG,
+	QUEUE_ORDERED_TAG		= QUEUE_ORDERED_BY_TAG |
+					  QUEUE_ORDERED_DO_BAR,
 	QUEUE_ORDERED_TAG_FLUSH		= QUEUE_ORDERED_TAG |
 					  QUEUE_ORDERED_DO_PREFLUSH |
 					  QUEUE_ORDERED_DO_POSTFLUSH,