[Yaffs] multithreading question

Hi!
It appears that Yaffs2 multithreading protection is on a very high level (mutex wrapping entire fread/fwrite calls).

My worry is that if a low-priority task does a large fwrite call, it could block high-priority tasks for a long time.
Is this an acknowledged problem? Are there comparable file systems that handle multithreading in a more granular way?

Regards,
Johan