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'm not really experienced on this by any means, but here's my take. If you look at glibc's implementation of sem_open, and sem_wait, it's really just creating a file in /dev/shm, mmap'ing a struct from it, and using atomic operations on it. If you want to access the named semaphore from user space, you will probably have to patch the tmpfs subsystem. However, I think this would be difficult, as it wouldn't be straightforward to determine if a file is meant to be a named semaphore.

An easier way would probably be to just reuse the kernel's semaphore implementation and have the kernel manage the semaphore for userspace processes. To do this, you would write a kernel module which you associate with a device file. Then define two ioctl's for the device file, one for wait, and one for post. Here is a good tutorial on writing kernel modules, including setting up a device file and adding I/O operations for it. http://www.freesoftwaremagazine.com/articles/drivers_linux. I don't know exactly how to implement an ioctl operation, but I think you can just assign a function to the ioctl member of the file_operations struct. Not sure what the function signature should be, but you could probably figure it out by digging around in the kernel source.