Shared semaphore between user and kernel spaces

Question

Short version

Is it possible to share a semaphore (or any other synchronization lock) between user space and kernel space? Named POSIX semaphores have kernel persi

Answer 1

I was thinking about ways that kernel and user land share things directly i.e. without syscall/copyin-out cost. One thing I remembered was the RDMA model where the kernel writes/reads directly from user space, with synchronization of course. You may want to explore that model and see if it works for your purpose.