md: restore ability of spare drives to spin down.

Some time ago we stopped the clean/active metadata updates
from being written to a 'spare' device in most cases so that
it could spin down and say spun down.  Device failure/removal
etc are still recorded on spares.

However commit 51d5668cb2e3fd1827a55 broke this 50% of the time,
depending on whether the event count is even or odd.
The change log entry said:

   This means that the alignment between 'odd/even' and
    'clean/dirty' might take a little longer to attain,

how ever the code makes no attempt to create that alignment, so it
could take arbitrarily long.

So when we find that clean/dirty is not aligned with odd/even,
force a second metadata-update immediately.  There are already cases
where a second metadata-update is needed immediately (e.g. when a
device fails during the metadata update).  We just piggy-back on that.

Reported-by: Joe Bryant <tenminjoe@yahoo.com>
Signed-off-by: NeilBrown <neilb@suse.de>
Cc: stable@kernel.org

1 files changed, 8 insertions, 2 deletions

diff --git a/drivers/md/md.c b/drivers/md/md.c
index b8a0fcfb1de1..fec4abcb9bb4 100644
--- a/drivers/md/md.c
+++ b/drivers/md/md.c
@@ -2150,12 +2150,18 @@ repeat:
                 if (!mddev->in_sync || mddev->recovery_cp != MaxSector) { /* not clean */
                         /* .. if the array isn't clean, an 'even' event must also go
                          * to spares. */
-                        if ((mddev->events&1)==0)
+                        if ((mddev->events&1)==0) {
                                 nospares = 0;
+                                sync_req = 2; /* force a second update to get the
+                                               * even/odd in sync */
+                        }
                 } else {
                         /* otherwise an 'odd' event must go to spares */
-                        if ((mddev->events&1))
+                        if ((mddev->events&1)) {
                                 nospares = 0;
+                                sync_req = 2; /* force a second update to get the
+                                               * even/odd in sync */
+                        }
                 }
         }