bcachefs: Ensure bch2_btree_node_get() calls relock() after unlock()

Fix a bug where bch2_btree_node_get() might call bch2_trans_unlock() (in
fill) without calling bch2_trans_relock(); this is a bug when it's done
in the core btree code.

Also, twea bch2_btree_node_mem_alloc() to drop btree locks before doing
a blocking memory allocation.

Signed-off-by: Kent Overstreet <[e-mail]>

fs/bcachefs/btree_cache.c

···
 			goto got_node;
 		}
 
-	b = __btree_node_mem_alloc(c, __GFP_NOWARN);
+	b = __btree_node_mem_alloc(c, GFP_NOWAIT|__GFP_NOWARN);
 	if (!b) {
 		mutex_unlock(&bc->lock);
+		bch2_trans_unlock(trans);
 		b = __btree_node_mem_alloc(c, GFP_KERNEL);
 		if (!b)
 			goto err;
···
 
 	mutex_unlock(&bc->lock);
 
-	if (btree_node_data_alloc(c, b, __GFP_NOWARN|GFP_KERNEL))
-		goto err;
+	if (btree_node_data_alloc(c, b, GFP_NOWAIT|__GFP_NOWARN)) {
+		bch2_trans_unlock(trans);
+		if (btree_node_data_alloc(c, b, GFP_KERNEL|__GFP_NOWARN))
+			goto err;
+	}
 
 	mutex_lock(&bc->lock);
 	bc->used++;
···
 	struct btree_cache *bc = &c->btree_cache;
 	struct btree *b;
 	struct bset_tree *t;
+	bool need_relock = false;
 	int ret;
 
 	EBUG_ON(level >= BTREE_MAX_DEPTH);
···
 		 */
 		b = bch2_btree_node_fill(trans, path, k, path->btree_id,
 					 level, lock_type, true);
+		need_relock = true;
 
 		/* We raced and found the btree node in the cache */
 		if (!b)
···
 
 		six_unlock_type(&b->c.lock, lock_type);
 		bch2_trans_unlock(trans);
+		need_relock = true;
 
 		bch2_btree_node_wait_on_read(b);
 
···
 		 * should_be_locked is not set on this path yet, so we need to
 		 * relock it specifically:
 		 */
-		if (trans) {
-			int ret = bch2_trans_relock(trans) ?:
-				bch2_btree_path_relock_intent(trans, path);
-			if (ret) {
-				BUG_ON(!trans->restarted);
-				return ERR_PTR(ret);
-			}
-		}
-
 		if (!six_relock_type(&b->c.lock, lock_type, seq))
 			goto retry;
+	}
+
+	if (unlikely(need_relock)) {
+		int ret = bch2_trans_relock(trans) ?:
+			bch2_btree_path_relock_intent(trans, path);
+		if (ret) {
+			six_unlock_type(&b->c.lock, lock_type);
+			return ERR_PTR(ret);
+		}
 	}
 
 	prefetch(b->aux_data);