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 ec23ca02f2fe..952aee04a68a 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