md: simplify updating of event count to sometimes avoid updating spares.

When updating the event count for a simple clean <-> dirty transition,
we try to avoid updating the spares so they can safely spin-down.
As the event_counts across an array must be +/- 1, this means
decrementing the event_count on a dirty->clean transition.
This is not always safe and we have to avoid the unsafe time.
We current do this with a misguided idea about it being safe or
not depending on whether the event_count is odd or even.  This
approach only works reliably in a few common instances, but easily
falls down.

So instead, simply keep internal state concerning whether it is safe
or not, and always assume it is not safe when an array is first
assembled.

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

1 files changed, 6 insertions, 20 deletions

diff --git a/drivers/md/md.c b/drivers/md/md.c
index fec4abcb9bb4..9ef21d9b8e27 100644
--- a/drivers/md/md.c
+++ b/drivers/md/md.c
@@ -2088,7 +2088,6 @@ static void sync_sbs(mddev_t * mddev, int nospares)
                 if (rdev->sb_events == mddev->events ||
                     (nospares &&
                      rdev->raid_disk < 0 &&
-                     (rdev->sb_events&1)==0 &&
                      rdev->sb_events+1 == mddev->events)) {
                         /* Don't update this superblock */
                         rdev->sb_loaded = 2;
@@ -2141,28 +2140,14 @@ repeat:
          * and 'events' is odd, we can roll back to the previous clean state */
         if (nospares
             && (mddev->in_sync && mddev->recovery_cp == MaxSector)
-            && (mddev->events & 1)
-            && mddev->events != 1)
+            && mddev->can_decrease_events
+            && mddev->events != 1) {
                 mddev->events--;
-        else {
+                mddev->can_decrease_events = 0;
+        } else {
                 /* otherwise we have to go forward and ... */
                 mddev->events ++;
-                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) {
-                                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)) {
-                                nospares = 0;
-                                sync_req = 2; /* force a second update to get the
-                                               * even/odd in sync */
-                        }
-                }
+                mddev->can_decrease_events = nospares;
         }
 
         if (!mddev->events) {
@@ -4606,6 +4591,7 @@ static void md_clean(mddev_t *mddev)
         mddev->layout = 0;
         mddev->max_disks = 0;
         mddev->events = 0;
+        mddev->can_decrease_events = 0;
         mddev->delta_disks = 0;
         mddev->new_level = LEVEL_NONE;
         mddev->new_layout = 0;