drbd: allow parallel promote/demote actions

We plan to use genl_family->parallel_ops = true in the future,
but need to review all possible interactions first.

For now, only selectively drop genl_lock() in drbd_set_role(),
instead serializing on our own internal resource->conf_update mutex.

We now can be promoted/demoted on many resources in parallel,
which may significantly improve cluster failover times
when fencing is required.
Signed-off-by: Philipp Reisner <philipp.reisner@linbit.com>
Signed-off-by: Lars Ellenberg <lars.ellenberg@linbit.com>
Signed-off-by: Jens Axboe <axboe@fb.com>

drivers/block/drbd/drbd_int.h

@@ -579,6 +579,7 @@ struct drbd_resource {
 	struct list_head resources;
 	struct res_opts res_opts;
 	struct mutex conf_update;	/* mutex for ready-copy-update of net_conf and disk_conf */
+	struct mutex adm_mutex;		/* mutex to serialize administrative requests */
 	spinlock_t req_lock;
 
 	unsigned susp:1;		/* IO suspended by user */

drivers/block/drbd/drbd_main.c

@@ -2570,6 +2570,7 @@ struct drbd_resource *drbd_create_resource(const char *name)
 	INIT_LIST_HEAD(&resource->connections);
 	list_add_tail_rcu(&resource->resources, &drbd_resources);
 	mutex_init(&resource->conf_update);
+	mutex_init(&resource->adm_mutex);
 	spin_lock_init(&resource->req_lock);
 	return resource;