block: Initial support for data-less (or empty) barrier support

This implements functionality to pass down or insert a barrier
in a queue, without having data attached to it. The ->prepare_flush_fn()
infrastructure from data barriers are reused to provide this
functionality.

Signed-off-by: Jens Axboe <jens.axboe@oracle.com>

1 files changed, 8 insertions, 0 deletions

diff --git a/block/elevator.c b/block/elevator.c
index ec23ca02f2f..952aee04a68 100644
--- a/block/elevator.c
+++ b/block/elevator.c
@@ -712,6 +712,14 @@ struct request *elv_next_request(struct request_queue *q)
 	int ret;
 
 	while ((rq = __elv_next_request(q)) != NULL) {
+		/*
+		 * Kill the empty barrier place holder, the driver must
+		 * not ever see it.
+		 */
+		if (blk_empty_barrier(rq)) {
+			end_queued_request(rq, 1);
+			continue;
+		}
 		if (!(rq->cmd_flags & REQ_STARTED)) {
 			/*
 			 * This is the first time the device driver