Locks are perfectly usable in filesystem code, but test_and_set_bit()/wait_on_bit() has lower overhead, so they'll get used as an optimization. This function is called on every metadata read, so the improved performance/scalability of raw atomics over locks can probably make a difference on fast storage.

Also the code used to use locks, and it wasn't any simpler: https://lore.kernel.org/linux-btrfs/20230503152441.1141019-2...