Every database management system that has half-decent SMP implementation and is portable to multiple platforms and has enough history that it is running on old

Every database management system that has half-decent SMP implementation and is portable to multiple platforms and has enough history that it is running on old busted kernels has already gotten a heavily tested, high performance spinlock mechanism. Including Oracle. Xorg is given as another example, not in being a database system, but having to deal with multiple platforms and busted stuff. Postgres as a project is very keen to offload tasks to the operating system when operating systems at large are not unacceptably slow or broken, and sometimes even when they are, but nobody has the resources to do anything about it. That's why it doesn't schedule its own writes using O_DIRECT, taking a hit in buffer copying from shared memory, unlike virtually every proprietary database.