bcachefs: Split out btree_node_rewrite_worker

This fixes a deadlock due to using btree_interior_update_worker for non
interior updates - async btree node rewrites were blocking, and then
blocking other interior updates.
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

fs/bcachefs/bcachefs.h

@@ -849,6 +849,8 @@ struct bch_fs {
 	struct workqueue_struct	*btree_interior_update_worker;
 	struct work_struct	btree_interior_update_work;
 
+	struct workqueue_struct	*btree_node_rewrite_worker;
+
 	struct list_head	pending_node_rewrites;
 	struct mutex		pending_node_rewrites_lock;
 

fs/bcachefs/btree_update_interior.c

@@ -2161,7 +2161,7 @@ void bch2_btree_node_rewrite_async(struct bch_fs *c, struct btree *b)
 		bch2_write_ref_get(c, BCH_WRITE_REF_node_rewrite);
 	}
 
-	queue_work(c->btree_interior_update_worker, &a->work);
+	queue_work(c->btree_node_rewrite_worker, &a->work);
 }
 
 void bch2_do_pending_node_rewrites(struct bch_fs *c)
@@ -2173,7 +2173,7 @@ void bch2_do_pending_node_rewrites(struct bch_fs *c)
 		list_del(&a->list);
 
 		bch2_write_ref_get(c, BCH_WRITE_REF_node_rewrite);
-		queue_work(c->btree_interior_update_worker, &a->work);
+		queue_work(c->btree_node_rewrite_worker, &a->work);
 	}
 	mutex_unlock(&c->pending_node_rewrites_lock);
 }
@@ -2510,6 +2510,8 @@ bch2_btree_roots_to_journal_entries(struct bch_fs *c,
 
 void bch2_fs_btree_interior_update_exit(struct bch_fs *c)
 {
+	if (c->btree_node_rewrite_worker)
+		destroy_workqueue(c->btree_node_rewrite_worker);
 	if (c->btree_interior_update_worker)
 		destroy_workqueue(c->btree_interior_update_worker);
 	mempool_exit(&c->btree_interior_update_pool);
@@ -2534,6 +2536,11 @@ int bch2_fs_btree_interior_update_init(struct bch_fs *c)
 	if (!c->btree_interior_update_worker)
 		return -BCH_ERR_ENOMEM_btree_interior_update_worker_init;
 
+	c->btree_node_rewrite_worker =
+		alloc_ordered_workqueue("btree_node_rewrite", WQ_UNBOUND);
+	if (!c->btree_node_rewrite_worker)
+		return -BCH_ERR_ENOMEM_btree_interior_update_worker_init;
+
 	if (mempool_init_kmalloc_pool(&c->btree_interior_update_pool, 1,
 				      sizeof(struct btree_update)))
 		return -BCH_ERR_ENOMEM_btree_interior_update_pool_init;