bcachefs: Simplify, fix bch2_backpointer_get_key()

 - backpointer_not_found() checks backpointers_no_use_write_buffer, no
   need to do it inbackpointer_get_key().

 - always use backpointer_get_node() for pointers to nodes:
   backpointer_get_key() was sometimes returning the key from the root
   node unlocked.
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

fs/bcachefs/backpointers.c

@@ -5,6 +5,7 @@
 #include "backpointers.h"
 #include "btree_cache.h"
 #include "btree_update.h"
+#include "btree_update_interior.h"
 #include "btree_write_buffer.h"
 #include "error.h"
 
@@ -226,6 +227,11 @@ static void backpointer_not_found(struct btree_trans *trans,
 	struct printbuf buf = PRINTBUF;
 	struct bpos bucket = bp_pos_to_bucket(c, bp_pos);
 
+	/*
+	 * If we're using the btree write buffer, the backpointer we were
+	 * looking at may have already been deleted - failure to find what it
+	 * pointed to is not an error:
+	 */
 	if (likely(!bch2_backpointers_no_use_write_buffer))
 		return;
 
@@ -256,56 +262,37 @@ struct bkey_s_c bch2_backpointer_get_key(struct btree_trans *trans,
 					 struct bch_backpointer bp,
 					 unsigned iter_flags)
 {
-	struct bch_fs *c = trans->c;
-	struct btree_root *r = bch2_btree_id_root(c, bp.btree_id);
-	struct bpos bucket = bp_pos_to_bucket(c, bp_pos);
-	struct bkey_s_c k;
-
-	bch2_trans_node_iter_init(trans, iter,
-				  bp.btree_id,
-				  bp.pos,
-				  0,
-				  min(bp.level, r->level),
-				  iter_flags);
-	k = bch2_btree_iter_peek_slot(iter);
-	if (bkey_err(k)) {
-		bch2_trans_iter_exit(trans, iter);
-		return k;
-	}
-
-	if (bp.level == r->level + 1)
-		k = bkey_i_to_s_c(&r->key);
-
-	if (k.k && extent_matches_bp(c, bp.btree_id, bp.level, k, bucket, bp))
-		return k;
-
-	bch2_trans_iter_exit(trans, iter);
+	if (likely(!bp.level)) {
+		struct bch_fs *c = trans->c;
+		struct bpos bucket = bp_pos_to_bucket(c, bp_pos);
+		struct bkey_s_c k;
+
+		bch2_trans_node_iter_init(trans, iter,
+					  bp.btree_id,
+					  bp.pos,
+					  0, 0,
+					  iter_flags);
+		k = bch2_btree_iter_peek_slot(iter);
+		if (bkey_err(k)) {
+			bch2_trans_iter_exit(trans, iter);
+			return k;
+		}
 
-	if (unlikely(bch2_backpointers_no_use_write_buffer)) {
-		if (bp.level) {
-			struct btree *b;
+		if (k.k && extent_matches_bp(c, bp.btree_id, bp.level, k, bucket, bp))
+			return k;
 
-			/*
-			 * If a backpointer for a btree node wasn't found, it may be
-			 * because it was overwritten by a new btree node that hasn't
-			 * been written out yet - backpointer_get_node() checks for
-			 * this:
-			 */
-			b = bch2_backpointer_get_node(trans, iter, bp_pos, bp);
-			if (!IS_ERR_OR_NULL(b))
-				return bkey_i_to_s_c(&b->key);
+		bch2_trans_iter_exit(trans, iter);
+		backpointer_not_found(trans, bp_pos, bp, k);
+		return bkey_s_c_null;
+	} else {
+		struct btree *b = bch2_backpointer_get_node(trans, iter, bp_pos, bp);
 
+		if (IS_ERR_OR_NULL(b)) {
 			bch2_trans_iter_exit(trans, iter);
-
-			if (IS_ERR(b))
-				return bkey_s_c_err(PTR_ERR(b));
-			return bkey_s_c_null;
+			return IS_ERR(b) ? bkey_s_c_err(PTR_ERR(b)) : bkey_s_c_null;
 		}
-
-		backpointer_not_found(trans, bp_pos, bp, k);
+		return bkey_i_to_s_c(&b->key);
 	}
-
-	return bkey_s_c_null;
 }
 
 struct btree *bch2_backpointer_get_node(struct btree_trans *trans,
@@ -329,6 +316,8 @@ struct btree *bch2_backpointer_get_node(struct btree_trans *trans,
 	if (IS_ERR(b))
 		goto err;
 
+	BUG_ON(b->c.level != bp.level - 1);
+
 	if (b && extent_matches_bp(c, bp.btree_id, bp.level,
 				   bkey_i_to_s_c(&b->key),
 				   bucket, bp))