bcachefs: Mark write locks before taking lock

six locks are unfair: while a thread is blocked trying to take a write
lock, new read locks will fail. The new deadlock cycle detector makes
use of our existing lock tracing, so we need to tell it we're holding a
write lock before we take the lock for it to work correctly.
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

fs/bcachefs/btree_locking.h

@@ -271,10 +271,15 @@ static inline void bch2_btree_node_lock_write(struct btree_trans *trans,
 	EBUG_ON(path->l[b->c.level].lock_seq != b->c.lock.state.seq);
 	EBUG_ON(!btree_node_intent_locked(path, b->c.level));
 
+	/*
+	 * six locks are unfair, and read locks block while a thread wants a
+	 * write lock: thus, we need to tell the cycle detector we have a write
+	 * lock _before_ taking the lock:
+	 */
+	mark_btree_node_locked_noreset(path, b->c.level, SIX_LOCK_write);
+
 	if (unlikely(!six_trylock_write(&b->c.lock)))
 		__bch2_btree_node_lock_write(trans, b);
-
-	mark_btree_node_locked_noreset(path, b->c.level, SIX_LOCK_write);
 }
 
 /* relock: */

fs/bcachefs/btree_update_leaf.c

@@ -817,6 +817,13 @@ static inline int trans_lock_write(struct btree_trans *trans)
 		if (same_leaf_as_prev(trans, i))
 			continue;
 
+		/*
+		 * six locks are unfair, and read locks block while a thread
+		 * wants a write lock: thus, we need to tell the cycle detector
+		 * we have a write lock _before_ taking the lock:
+		 */
+		mark_btree_node_locked_noreset(i->path, i->level, SIX_LOCK_write);
+
 		if (!six_trylock_write(&insert_l(i)->b->c.lock)) {
 			if (have_conflicting_read_lock(trans, i->path))
 				goto fail;
@@ -828,13 +835,13 @@ static inline int trans_lock_write(struct btree_trans *trans)
 			BUG_ON(ret);
 		}
 
-		mark_btree_node_locked_noreset(i->path, i->level, SIX_LOCK_write);
-
 		bch2_btree_node_prep_for_write(trans, i->path, insert_l(i)->b);
 	}
 
 	return 0;
 fail:
+	mark_btree_node_locked_noreset(i->path, i->level, SIX_LOCK_intent);
+
 	while (--i >= trans->updates) {
 		if (same_leaf_as_prev(trans, i))
 			continue;