md: Ensure no IO request to get md device before it is properly initialised.

When an md device is in the process of coming on line it is possible
for an IO request (typically a partition table probe) to get through
before the array is fully initialised, which can cause unexpected
behaviour (e.g. a crash).

So explicitly record when the array is ready for IO and don't allow IO
through until then.

There is no possibility for a similar problem when the array is going
off-line as there must only be one 'open' at that time, and it is busy
off-lining the array and so cannot send IO requests.  So no memory
barrier is needed in md_stop()

This has been a bug since commit 409c57f3801 in 2.6.30 which
introduced md_make_request.  Before then, each personality would
register its own make_request_fn when it was ready.
This is suitable for any stable kernel from 2.6.30.y onwards.

Cc: <stable@kernel.org>
Signed-off-by: NeilBrown <neilb@suse.de>
Reported-by:  "Hawrylewicz Czarnowski, Przemyslaw" <przemyslaw.hawrylewicz.czarnowski@intel.com>

drivers/md/md.c

@@ -288,10 +288,12 @@ static int md_make_request(struct request_queue *q, struct bio *bio)
 	int rv;
 	int cpu;
 
-	if (mddev == NULL || mddev->pers == NULL) {
+	if (mddev == NULL || mddev->pers == NULL
+	    || !mddev->ready) {
 		bio_io_error(bio);
 		return 0;
 	}
+	smp_rmb(); /* Ensure implications of  'active' are visible */
 	rcu_read_lock();
 	if (mddev->suspended) {
 		DEFINE_WAIT(__wait);
@@ -4564,7 +4566,8 @@ int md_run(mddev_t *mddev)
 	mddev->safemode_timer.data = (unsigned long) mddev;
 	mddev->safemode_delay = (200 * HZ)/1000 +1; /* 200 msec delay */
 	mddev->in_sync = 1;
-
+	smp_wmb();
+	mddev->ready = 1;
 	list_for_each_entry(rdev, &mddev->disks, same_set)
 		if (rdev->raid_disk >= 0) {
 			char nm[20];
@@ -4725,6 +4728,7 @@ EXPORT_SYMBOL_GPL(md_stop_writes);
 
 void md_stop(mddev_t *mddev)
 {
+	mddev->ready = 0;
 	mddev->pers->stop(mddev);
 	if (mddev->pers->sync_request && mddev->to_remove == NULL)
 		mddev->to_remove = &md_redundancy_group;

drivers/md/md.h

@@ -148,7 +148,8 @@ struct mddev_s
 						       * are happening, so run/
 						       * takeover/stop are not safe
 						       */
-
+	int				ready; /* See when safe to pass 
+						* IO requests down */
 	struct gendisk			*gendisk;
 
 	struct kobject			kobj;