md: make merge_bvec_fn more robust in face of personality changes.

There is no locking around calls to merge_bvec_fn(), so
it is possible that calls which coincide with a level (or personality)
change could go wrong.

So create a central dispatch point for these functions and use
rcu_read_lock().
If the array is suspended, reject any merge that can be rejected.
If not, we know it is safe to call the function.

Signed-off-by: NeilBrown <neilb@suse.de>

1 files changed, 4 insertions, 0 deletions

diff --git a/drivers/md/md.h b/drivers/md/md.h
index f2602280fac1..bee5b852c33f 100644
--- a/drivers/md/md.h
+++ b/drivers/md/md.h
@@ -499,6 +499,10 @@ struct md_personality
         /* congested implements bdi.congested_fn().
          * Will not be called while array is 'suspended' */
         int (*congested)(struct mddev *mddev, int bits);
+        /* mergeable_bvec is use to implement ->merge_bvec_fn */
+        int (*mergeable_bvec)(struct mddev *mddev,
+                              struct bvec_merge_data *bvm,
+                              struct bio_vec *biovec);
 };
 
 struct md_sysfs_entry {