[PATCH 1/2] block: Introduce barrier counters

[Jan Kara]

Introduce a barrier counters to the block device that are incremented each time
before we send a barrier and after the barrier is completed.  Filesystems can
then use this counter to verify whether they need to issue a barrier to flush
file's data during fsync.

Signed-off-by: Jan Kara <jack@xxxxxxx>
---
 block/blk-core.c   |    5 +++++
 fs/bio.c           |    7 +++++++
 include/linux/fs.h |    4 ++++
 3 files changed, 16 insertions(+), 0 deletions(-)

diff --git a/block/blk-core.c b/block/blk-core.c
index f0640d7..6fc58e6 100644
--- a/block/blk-core.c
+++ b/block/blk-core.c
@@ -1600,6 +1600,11 @@ void submit_bio(int rw, struct bio *bio)
 				bdevname(bio->bi_bdev, b));
 		}
 	}
+	if (rw & (1 << BIO_RW_BARRIER)) {
+		atomic_inc(&bio->bi_bdev->bd_barriers_sent);
+		/* Make sure counter update is seen before IO happens */
+		smp_mb__after_atomic_inc();
+	}
 
 	generic_make_request(bio);
 }
diff --git a/fs/bio.c b/fs/bio.c
index e7bf6ca..59f01cb 100644
--- a/fs/bio.c
+++ b/fs/bio.c
@@ -1427,6 +1427,13 @@ void bio_endio(struct bio *bio, int error)
 	else if (!test_bit(BIO_UPTODATE, &bio->bi_flags))
 		error = -EIO;
 
+	/*
+	 * Increment even in case of error so that sent and completed
+	 * counters don't get out of sync
+	 */
+	if (bio_rw_flagged(bio, BIO_RW_BARRIER))
+		atomic_inc(&bio->bi_bdev->bd_barriers_completed);
+
 	if (bio->bi_end_io)
 		bio->bi_end_io(bio, error);
 }
diff --git a/include/linux/fs.h b/include/linux/fs.h
index 68ca1b0..4a6c91d 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -654,6 +654,10 @@ struct block_device {
 	void *			bd_claiming;
 	void *			bd_holder;
 	int			bd_holders;
+	/* Incremented each time before we send a barrier */
+	atomic_t		bd_barriers_sent;
+	/* Incremented each time after a barrier request completes */
+	atomic_t		bd_barriers_completed;
 #ifdef CONFIG_SYSFS
 	struct list_head	bd_holder_list;
 #endif
-- 
1.6.4.2


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Content-Disposition: attachment; filename="0002-ext4-Send-barriers-on-fsync-only-when-needed.patch"