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>

2 files changed, 18 insertions, 2 deletions

diff --git a/include/linux/bio.h b/include/linux/bio.h
index 089a8bc55dd4..4da441337d6e 100644
--- a/include/linux/bio.h
+++ b/include/linux/bio.h
@@ -176,13 +176,28 @@ struct bio {
 #define bio_offset(bio)         bio_iovec((bio))->bv_offset
 #define bio_segments(bio)       ((bio)->bi_vcnt - (bio)->bi_idx)
 #define bio_sectors(bio)        ((bio)->bi_size >> 9)
-#define bio_cur_sectors(bio)    (bio_iovec(bio)->bv_len >> 9)
-#define bio_data(bio)           (page_address(bio_page((bio))) + bio_offset((bio)))
 #define bio_barrier(bio)        ((bio)->bi_rw & (1 << BIO_RW_BARRIER))
 #define bio_sync(bio)           ((bio)->bi_rw & (1 << BIO_RW_SYNC))
 #define bio_failfast(bio)       ((bio)->bi_rw & (1 << BIO_RW_FAILFAST))
 #define bio_rw_ahead(bio)       ((bio)->bi_rw & (1 << BIO_RW_AHEAD))
 #define bio_rw_meta(bio)        ((bio)->bi_rw & (1 << BIO_RW_META))
+#define bio_empty_barrier(bio)  (bio_barrier(bio) && !(bio)->bi_size)
+
+static inline unsigned int bio_cur_sectors(struct bio *bio)
+{
+        if (bio->bi_vcnt)
+                return bio_iovec(bio)->bv_len >> 9;
+
+        return 0;
+}
+
+static inline void *bio_data(struct bio *bio)
+{
+        if (bio->bi_vcnt)
+                return page_address(bio_page(bio)) + bio_offset(bio);
+
+        return NULL;
+}
 
 /*
  * will die
diff --git a/include/linux/blkdev.h b/include/linux/blkdev.h
index 610967992ddb..fb2ff749dc1f 100644
--- a/include/linux/blkdev.h
+++ b/include/linux/blkdev.h
@@ -540,6 +540,7 @@ enum {
 #define blk_barrier_rq(rq)      ((rq)->cmd_flags & REQ_HARDBARRIER)
 #define blk_fua_rq(rq)          ((rq)->cmd_flags & REQ_FUA)
 #define blk_bidi_rq(rq)         ((rq)->next_rq != NULL)
+#define blk_empty_barrier(rq)   (blk_barrier_rq(rq) && blk_fs_request(rq) && !(rq)->hard_nr_sectors)
 
 #define list_entry_rq(ptr)      list_entry((ptr), struct request, queuelist)