md: never update metadata when array is read-only.

Normally we don't even try to update the metadata if
the array is read-only.  However future patches
will increase the number of things that can happen on a read-only
array, so it is safest to explicitly disable this.

Every time that mddev->ro is set to 0, either
 - md_update_sb will be called again (at least if MD_CHANGE_DEVS
   is set) or
 - the mddev->thread is scheduled, which will also run
   md_update_sb if needed.

So this is safe: if the array ever become read-write the
metadata will be updated.

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

1 files changed, 5 insertions, 0 deletions

diff --git a/drivers/md/md.c b/drivers/md/md.c
index aeceedfc530b..c6e44a836a53 100644
--- a/drivers/md/md.c
+++ b/drivers/md/md.c
@@ -2411,6 +2411,11 @@ static void md_update_sb(struct mddev * mddev, int force_change)
         int nospares = 0;
         int any_badblocks_changed = 0;
 
+        if (mddev->ro) {
+                if (force_change)
+                        set_bit(MD_CHANGE_DEVS, &mddev->flags);
+                return;
+        }
 repeat:
         /* First make sure individual recovery_offsets are correct */
         rdev_for_each(rdev, mddev) {