bcachefs: push rcu lock down into bch2_target_to_mask()

We have one caller that cycles the rcu lock solely for this call
(via target_rw_devs()), and we'd like to add another. Simplify
things by pushing the rcu lock down into bch2_target_to_mask(),
similar to how bch2_dev_in_target() works.
Signed-off-by: Brian Foster <bfoster@redhat.com>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

fs/bcachefs/alloc_foreground.c

@@ -934,9 +934,7 @@ static int __open_bucket_add_buckets(struct btree_trans *trans,
 	unsigned i;
 	int ret;
 
-	rcu_read_lock();
 	devs = target_rw_devs(c, wp->data_type, target);
-	rcu_read_unlock();
 
 	/* Don't allocate from devices we already have pointers to: */
 	for (i = 0; i < devs_have->nr; i++)

fs/bcachefs/disk_groups.c

@@ -208,26 +208,36 @@ int bch2_sb_disk_groups_to_cpu(struct bch_fs *c)
 const struct bch_devs_mask *bch2_target_to_mask(struct bch_fs *c, unsigned target)
 {
 	struct target t = target_decode(target);
+	struct bch_devs_mask *devs;
+
+	rcu_read_lock();
 
 	switch (t.type) {
 	case TARGET_NULL:
-		return NULL;
+		devs = NULL;
+		break;
 	case TARGET_DEV: {
 		struct bch_dev *ca = t.dev < c->sb.nr_devices
 			? rcu_dereference(c->devs[t.dev])
 			: NULL;
-		return ca ? &ca->self : NULL;
+		devs = ca ? &ca->self : NULL;
+		break;
 	}
 	case TARGET_GROUP: {
 		struct bch_disk_groups_cpu *g = rcu_dereference(c->disk_groups);
 
-		return g && t.group < g->nr && !g->entries[t.group].deleted
+		devs = g && t.group < g->nr && !g->entries[t.group].deleted
 			? &g->entries[t.group].devs
 			: NULL;
+		break;
 	}
 	default:
 		BUG();
 	}
+
+	rcu_read_unlock();
+
+	return devs;
 }
 
 bool bch2_dev_in_target(struct bch_fs *c, unsigned dev, unsigned target)