Re: [PATCH] Lightweight userspace semaphores...

[Alan Cox]

> 	fd = sem_initialize();
> 	mmap(fd, ...)
> 	..
> 	munmap(..)
> 
> which gives you a handle for the semaphore.

	fd = open("/dev/shm/sem....");
	mmap(fd, ...)

	munmap(..)

That lets the kernel decide what it wants to do and provide as the back
end. It allows you to think about things like mremap and growing/shrinking
the object. And finally /dev/sem looks suspiciously like a quick tweak
to /dev/shm..

> And make the initial mmap() only do a limited number of pages, so that
> people don't start trying to allocate tons of memory this way.-

If it uses the /dev/shm world then anyone running a kernel with the new
patches for resource accounting is still safe, and anyone else is still
simply going to find their shm areas hitting swap under extreme load (which
is ideal)