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>
diff --git a/block/elevator.c b/block/elevator.c
index ec23ca0..952aee0 100644
--- a/block/elevator.c
+++ b/block/elevator.c
@@ -712,6 +712,14 @@
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