drbd: allow write-ordering policy to be bumped up again

Previously, once you disabled flushes as a means of enforcing
write-ordering, you'd need to detach/re-attach to enable them again.

Allow drbdsetup disk-options to re-enable previously disabled
write-ordering policy options at runtime.

While at it fix RCU in drbd_bump_write_ordering()
max_allowed_wo() uses rcu_dereference, therefore it must
be called within rcu_read_lock()/rcu_read_unlock()
Signed-off-by: Philipp Reisner <philipp.reisner@linbit.com>
Signed-off-by: Lars Ellenberg <lars.ellenberg@linbit.com>

drivers/block/drbd/drbd_nl.c

@@ -1294,6 +1294,13 @@ static unsigned int drbd_al_extents_max(struct drbd_backing_dev *bdev)
 	return (al_size_4k - 1) * AL_CONTEXT_PER_TRANSACTION;
 }
 
+static bool write_ordering_changed(struct disk_conf *a, struct disk_conf *b)
+{
+	return	a->disk_barrier != b->disk_barrier ||
+		a->disk_flushes != b->disk_flushes ||
+		a->disk_drain != b->disk_drain;
+}
+
 int drbd_adm_disk_opts(struct sk_buff *skb, struct genl_info *info)
 {
 	struct drbd_config_context adm_ctx;
@@ -1400,6 +1407,7 @@ int drbd_adm_disk_opts(struct sk_buff *skb, struct genl_info *info)
 	else
 		set_bit(MD_NO_FUA, &device->flags);
 
+	if (write_ordering_changed(old_disk_conf, new_disk_conf))
 		drbd_bump_write_ordering(device->resource, NULL, WO_bdev_flush);
 
 	drbd_md_sync(device);

drivers/block/drbd/drbd_receiver.c

@@ -1290,6 +1290,7 @@ void drbd_bump_write_ordering(struct drbd_resource *resource, struct drbd_backin
 	};
 
 	pwo = resource->write_ordering;
+	if (wo != WO_bdev_flush)
 		wo = min(pwo, wo);
 	rcu_read_lock();
 	idr_for_each_entry(&resource->devices, device, vnr) {
@@ -1300,11 +1301,12 @@ void drbd_bump_write_ordering(struct drbd_resource *resource, struct drbd_backin
 			put_ldev(device);
 		}
 	}
-	rcu_read_unlock();
 
 	if (bdev)
 		wo = max_allowed_wo(bdev, wo);
 
+	rcu_read_unlock();
+
 	resource->write_ordering = wo;
 	if (pwo != resource->write_ordering || wo == WO_bdev_flush)
 		drbd_info(resource, "Method to ensure write ordering: %s\n", write_ordering_str[resource->write_ordering]);