[BLOCK] reimplement handling of barrier request

Reimplement handling of barrier requests.

* Flexible handling to deal with various capabilities of
  target devices.
* Retry support for falling back.
* Tagged queues which don't support ordered tag can do ordered.

Signed-off-by: Tejun Heo <htejun@gmail.com>
Signed-off-by: Jens Axboe <axboe@suse.de>

1 files changed, 1 insertions, 0 deletions

diff --git a/include/linux/elevator.h b/include/linux/elevator.h
index a74c27e460ba..fb80fa44c4dd 100644
--- a/include/linux/elevator.h
+++ b/include/linux/elevator.h
@@ -130,6 +130,7 @@ extern int elv_try_last_merge(request_queue_t *, struct bio *);
 #define ELEVATOR_INSERT_FRONT   1
 #define ELEVATOR_INSERT_BACK    2
 #define ELEVATOR_INSERT_SORT    3
+#define ELEVATOR_INSERT_REQUEUE 4
 
 /*
  * return values from elevator_may_queue_fn