md/raid5: close recently introduced race in stripe_head management.

As release_stripe and __release_stripe decrement ->count and then
manipulate ->lru both under ->device_lock, it is important that
get_active_stripe() increments ->count and clears ->lru also under
->device_lock.

However we currently list_del_init ->lru under the lock, but increment
the ->count outside the lock.  This can lead to races and list
corruption.

So move the atomic_inc(&sh->count) up inside the ->device_lock
protected region.

Note that we still increment ->count without device lock in the case
where get_free_stripe() was called, and in fact don't take
->device_lock at all in that path.
This is safe because if the stripe_head can be found by
get_free_stripe, then the hash lock assures us the no-one else could
possibly be calling release_stripe() at the same time.

Fixes: 566c09c53455d7c4f1130928ef8071da1a24ea65
Cc: stable@vger.kernel.org (3.13)
Reported-and-tested-by: Ian Kumlien <ian.kumlien@gmail.com>
Signed-off-by: NeilBrown <neilb@suse.de>

drivers/md/raid5.c

@@ -675,8 +675,10 @@ get_active_stripe(struct r5conf *conf, sector_t sector,
 					 || !conf->inactive_blocked),
 					*(conf->hash_locks + hash));
 				conf->inactive_blocked = 0;
-			} else
+			} else {
 				init_stripe(sh, sector, previous);
+				atomic_inc(&sh->count);
+			}
 		} else {
 			spin_lock(&conf->device_lock);
 			if (atomic_read(&sh->count)) {
@@ -695,13 +697,11 @@ get_active_stripe(struct r5conf *conf, sector_t sector,
 					sh->group = NULL;
 				}
 			}
+			atomic_inc(&sh->count);
 			spin_unlock(&conf->device_lock);
 		}
 	} while (sh == NULL);
 
-	if (sh)
-		atomic_inc(&sh->count);
-
 	spin_unlock_irq(conf->hash_locks + hash);
 	return sh;
 }