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 <[e-mail]>

fs/bcachefs/bcachefs.h

···
 	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

···
 		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)
···
 		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);
 }
···
 
 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);
···
 	c->btree_interior_update_worker =
 		alloc_workqueue("btree_update", WQ_UNBOUND|WQ_MEM_RECLAIM, 8);
 	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,