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, 0 deletions

diff --git a/drivers/md/md.h b/drivers/md/md.h
index a536f5458097..7ab5ea155452 100644
--- a/drivers/md/md.h
+++ b/drivers/md/md.h
@@ -150,6 +150,12 @@ struct mddev_s
 	int				external_size; /* size managed
 							* externally */
 	__u64				events;
+	/* If the last 'event' was simply a clean->dirty transition, and
+	 * we didn't write it to the spares, then it is safe and simple
+	 * to just decrement the event count on a dirty->clean transition.
+	 * So we record that possibility here.
+	 */
+	int				can_decrease_events;
 
 	char				uuid[16];