md: manage redundancy group in sysfs when changing level.

Some levels expect the 'redundancy group' to be present,
others don't.
So when we change level of an array we might need to
add or remove this group.

This requires fixing up the current practice of overloading ->private
to indicate (when ->pers == NULL) that something needs to be removed.
So create a new ->to_remove to fill that role.

When changing levels, we may need to add or remove attributes.  When
changing RAID5 -> RAID6, we both add and remove the same thing.  It is
important to catch this and optimise it out as the removal is delayed
until a lock is released, so trying to add immediately would cause
problems.


Cc: stable@kernel.org
Signed-off-by: NeilBrown <neilb@suse.de>

1 files changed, 1 insertions, 0 deletions

diff --git a/drivers/md/md.h b/drivers/md/md.h
index 8e4c75c00d4..722f5dfe195 100644
--- a/drivers/md/md.h
+++ b/drivers/md/md.h
@@ -305,6 +305,7 @@ struct mddev_s
         atomic_t                        max_corr_read_errors; /* max read retries */
         struct list_head                all_mddevs;
 
+        struct attribute_group          *to_remove;
         /* Generic barrier handling.
          * If there is a pending barrier request, all other
          * writes are blocked while the devices are flushed.