[PATCH] md: Improve detection of lack of barrier support in raid1

Move the test for 'do barrier work' down a bit so that if the first write to a
raid1 is a BIO_RW_BARRIER write, the checking done by superblock writes will
cause the right thing to happen.

Signed-off-by: Neil Brown <neilb@suse.de>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

1 files changed, 11 insertions, 5 deletions

diff --git a/drivers/md/raid1.c b/drivers/md/raid1.c
index 532d04077301..b8c13c897ccb 100644
--- a/drivers/md/raid1.c
+++ b/drivers/md/raid1.c
@@ -753,18 +753,24 @@ static int make_request(request_queue_t *q, struct bio * bio)
         const int rw = bio_data_dir(bio);
         int do_barriers;
 
-        if (unlikely(!mddev->barriers_work && bio_barrier(bio))) {
-                bio_endio(bio, bio->bi_size, -EOPNOTSUPP);
-                return 0;
-        }
-
         /*
          * Register the new request and wait if the reconstruction
          * thread has put up a bar for new requests.
          * Continue immediately if no resync is active currently.
+         * We test barriers_work *after* md_write_start as md_write_start
+         * may cause the first superblock write, and that will check out
+         * if barriers work.
          */
+
         md_write_start(mddev, bio); /* wait on superblock update early */
 
+        if (unlikely(!mddev->barriers_work && bio_barrier(bio))) {
+                if (rw == WRITE)
+                        md_write_end(mddev);
+                bio_endio(bio, bio->bi_size, -EOPNOTSUPP);
+                return 0;
+        }
+
         wait_barrier(conf);
 
         disk_stat_inc(mddev->gendisk, ios[rw]);