block: BARRIER request should imply SYNC

A barrier request should by defintion have priority in get_request
and let the queue be unplugged immediately as it's blocking all forward
progress due to the queue draining.

Most filesystems already get this implicitly by the way how submit_bh
treats the buffer_ordered flag, and gfs2 sets it explicitly.  But btrfs
and XFS are still forgetting to set the flag, as is blkdev_issue_flush
and some places in DM/MD.

For XFS on metadata heavy workloads this gives a consistent speedup
in the 2-3% range.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <jaxboe@fusionio.com>

1 files changed, 2 insertions, 2 deletions

diff --git a/include/linux/fs.h b/include/linux/fs.h
index 68ca1b0491af..598878831497 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -136,7 +136,7 @@ struct inodes_stat_t {
  * SWRITE_SYNC
  * SWRITE_SYNC_PLUG     Like WRITE_SYNC/WRITE_SYNC_PLUG, but locks the buffer.
  *                      See SWRITE.
- * WRITE_BARRIER        Like WRITE, but tells the block layer that all
+ * WRITE_BARRIER        Like WRITE_SYNC, but tells the block layer that all
  *                      previously submitted writes must be safely on storage
  *                      before this one is started. Also guarantees that when
  *                      this write is complete, it itself is also safely on
@@ -159,7 +159,7 @@ struct inodes_stat_t {
 #define SWRITE_SYNC_PLUG        \
                         (SWRITE | (1 << BIO_RW_SYNCIO) | (1 << BIO_RW_NOIDLE))
 #define SWRITE_SYNC     (SWRITE_SYNC_PLUG | (1 << BIO_RW_UNPLUG))
-#define WRITE_BARRIER   (WRITE | (1 << BIO_RW_BARRIER))
+#define WRITE_BARRIER   (WRITE_SYNC | (1 << BIO_RW_BARRIER))
 
 /*
  * These aren't really reads or writes, they pass down information about