nvme: avoid race in shutdown namespace removal

When we remove the siblings entry, we update ns->head->list, hence we
can't separate the removal and test for being empty. They have to be
in the same critical section to avoid a race.

To avoid breaking the refcounting imbalance again, add a list empty
check to nvme_find_ns_head.

Fixes: 5396fdac ("nvme: fix refcounting imbalance when all paths are down")
Signed-off-by: Daniel Wagner <dwagner@suse.de>
Reviewed-by: Hannes Reinecke <hare@suse.de>
Tested-by: Hannes Reinecke <hare@suse.de>
Signed-off-by: Christoph Hellwig <hch@lst.de>

drivers/nvme/host/core.c

@@ -3524,7 +3524,9 @@ static struct nvme_ns_head *nvme_find_ns_head(struct nvme_subsystem *subsys,
 	lockdep_assert_held(&subsys->lock);
 
 	list_for_each_entry(h, &subsys->nsheads, entry) {
-		if (h->ns_id == nsid && nvme_tryget_ns_head(h))
+		if (h->ns_id != nsid)
+			continue;
+		if (!list_empty(&h->list) && nvme_tryget_ns_head(h))
 			return h;
 	}
 
@@ -3843,6 +3845,10 @@ static void nvme_ns_remove(struct nvme_ns *ns)
 
 	mutex_lock(&ns->ctrl->subsys->lock);
 	list_del_rcu(&ns->siblings);
+	if (list_empty(&ns->head->list)) {
+		list_del_init(&ns->head->entry);
+		last_path = true;
+	}
 	mutex_unlock(&ns->ctrl->subsys->lock);
 
 	/* guarantee not available in head->list */
@@ -3863,13 +3869,6 @@ static void nvme_ns_remove(struct nvme_ns *ns)
 	list_del_init(&ns->list);
 	up_write(&ns->ctrl->namespaces_rwsem);
 
-	/* Synchronize with nvme_init_ns_head() */
-	mutex_lock(&ns->head->subsys->lock);
-	if (list_empty(&ns->head->list)) {
-		list_del_init(&ns->head->entry);
-		last_path = true;
-	}
-	mutex_unlock(&ns->head->subsys->lock);
 	if (last_path)
 		nvme_mpath_shutdown_disk(ns->head);
 	nvme_put_ns(ns);