nvme: change locking for the per-subsystem controller list

Life becomes a lot simpler if we just use the global
nvme_subsystems_lock to protect this list.  Given that it is only
accessed during controller probing and removal that isn't a scalability
problem either.
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Chaitanya Kulkarni <chaitanya.kulkarni@wdc.com>
Reviewed-by: Hannes Reinecke <hare@suse.com>

drivers/nvme/host/core.c

@@ -2346,13 +2346,13 @@ static int nvme_active_ctrls(struct nvme_subsystem *subsys)
 	int count = 0;
 	struct nvme_ctrl *ctrl;
 
-	mutex_lock(&subsys->lock);
+	lockdep_assert_held(&nvme_subsystems_lock);
+
 	list_for_each_entry(ctrl, &subsys->ctrls, subsys_entry) {
 		if (ctrl->state != NVME_CTRL_DELETING &&
 		    ctrl->state != NVME_CTRL_DEAD)
 			count++;
 	}
-	mutex_unlock(&subsys->lock);
 
 	return count;
 }
@@ -2394,6 +2394,9 @@ static int nvme_init_subsystem(struct nvme_ctrl *ctrl, struct nvme_id_ctrl *id)
 	mutex_lock(&nvme_subsystems_lock);
 	found = __nvme_find_get_subsystem(subsys->subnqn);
 	if (found) {
+		__nvme_release_subsystem(subsys);
+		subsys = found;
+
 		/*
 		 * Verify that the subsystem actually supports multiple
 		 * controllers, else bail out.
@@ -2402,14 +2405,10 @@ static int nvme_init_subsystem(struct nvme_ctrl *ctrl, struct nvme_id_ctrl *id)
 		    nvme_active_ctrls(found) && !(id->cmic & (1 << 1))) {
 			dev_err(ctrl->device,
 				"ignoring ctrl due to duplicate subnqn (%s).\n",
-				found->subnqn);
-			nvme_put_subsystem(found);
+				subsys->subnqn);
 			ret = -EINVAL;
-			goto out_unlock;
+			goto out_put_subsystem;
 		}
-
-		__nvme_release_subsystem(subsys);
-		subsys = found;
 	} else {
 		ret = device_add(&subsys->dev);
 		if (ret) {
@@ -2421,23 +2420,20 @@ static int nvme_init_subsystem(struct nvme_ctrl *ctrl, struct nvme_id_ctrl *id)
 		list_add_tail(&subsys->entry, &nvme_subsystems);
 	}
 
-	ctrl->subsys = subsys;
-	mutex_unlock(&nvme_subsystems_lock);
-
 	if (sysfs_create_link(&subsys->dev.kobj, &ctrl->device->kobj,
 			dev_name(ctrl->device))) {
 		dev_err(ctrl->device,
 			"failed to create sysfs link from subsystem.\n");
-		/* the transport driver will eventually put the subsystem */
-		return -EINVAL;
+		goto out_put_subsystem;
 	}
 
-	mutex_lock(&subsys->lock);
+	ctrl->subsys = subsys;
 	list_add_tail(&ctrl->subsys_entry, &subsys->ctrls);
-	mutex_unlock(&subsys->lock);
-
+	mutex_unlock(&nvme_subsystems_lock);
 	return 0;
 
+out_put_subsystem:
+	nvme_put_subsystem(subsys);
 out_unlock:
 	mutex_unlock(&nvme_subsystems_lock);
 	put_device(&subsys->dev);
@@ -3694,10 +3690,10 @@ static void nvme_free_ctrl(struct device *dev)
 	__free_page(ctrl->discard_page);
 
 	if (subsys) {
-		mutex_lock(&subsys->lock);
+		mutex_lock(&nvme_subsystems_lock);
 		list_del(&ctrl->subsys_entry);
-		mutex_unlock(&subsys->lock);
 		sysfs_remove_link(&subsys->dev.kobj, dev_name(ctrl->device));
+		mutex_unlock(&nvme_subsystems_lock);
 	}
 
 	ctrl->ops->free_ctrl(ctrl);