Semaphores outside the kernel
Jason Gait · ACM SIGPLAN Notices · 1984
We present three new implementations of semaphores for V7 UNIX. None require kernel modifications. The canonical UNIX semaphore method uses link O in a spin loop. The first new method replaces the canonical spin loop with a software trap mechanism, the second provides additional enhancements in the form of a pool of semaphores and explicit quene management, and the third is a distributed protocol extending to the ability for synchronizing in network environments.