Documentation/block/barrier.txt is not in sync with the actual code: -...

typedef void (prepare_flush_fn)(request_queue_t *q, struct request *rq);

int blk_queue_ordered(request_queue_t *q, unsigned ordered,

		      prepare_flush_fn *prepare_flush_fn,

		      unsigned gfp_mask);

int blk_queue_ordered_locked(request_queue_t *q, unsigned ordered,

			     prepare_flush_fn *prepare_flush_fn,

			     unsigned gfp_mask);

The only difference between the two functions is whether or not the

caller is holding q->queue_lock on entry.  The latter expects the

caller is holding the lock.

		      prepare_flush_fn *prepare_flush_fn);

@q			: the queue in question

@ordered		: the ordered mode the driver/device supports

@prepare_flush_fn	: this function should prepare @rq such that it

			  flushes cache to physical medium when executed

@gfp_mask		: gfp_mask used when allocating data structures

			  for ordered processing

For example, SCSI disk driver's prepare_flush_fn looks like the

following.

static void sd_prepare_flush(request_queue_t *q, struct request *rq)

{

	memset(rq->cmd, 0, sizeof(rq->cmd));

	rq->flags |= REQ_BLOCK_PC;

	rq->cmd_type = REQ_TYPE_BLOCK_PC;

	rq->timeout = SD_TIMEOUT;

	rq->cmd[0] = SYNCHRONIZE_CACHE;

	rq->cmd_len = 10;

}

The following seven ordered modes are supported.  The following table