> This stuff was all well understood four decades ago. Much of it was forgotten outside the mainframe world, because threads and multiprocessors didn't make it to microprocessors for several more decades.

Here's an important distinction to make: this stuff was well understood in theory, but the practice is a bit different.

Semaphores are a neat theoretical concept but not a very good practical parallel programming paradigm. Semaphores are easy to reason about when writing proofs by induction that a parallel programming algorithm is working correctly, which is why they are still at the core of parallel programming education.

But when writing practical multithreaded programs, mutexes and condition variables are a lot more practical. Typically each mutex is coupled with one or more condition variables to wait/signal for conditions such as "queue not full" and "queue not empty". Incrementing and decrementing numeric counters is a very clumsy way to maintain a state of any kind. Implementing a semaphore requires grabbing a spinlock and doing this several times is unnecessary when you could just grab one mutex, check for the appropriate condition(s) and then wait/signal on the correct condition.

It is rather unfortunate that parallel programming is still primarily being taught in the theoretical manner (at least I was) using primarily semaphores, leaving too little emphasis on the practical implementation. It is important to understand the theory and know how to do the proofs but it is equally important to apply this into practice.

Every time I see someone "implement" a semaphore using a pthread mutex and condition variable, I cry a little. I've seen this several times in production code.

> It's very simple. That's the real use case for P and V. Linus' note indicates that in 1999 he didn't know this.

I'm pretty sure Linus was joking and he did understand what semaphores are used for. Every computer science curriculum has a course on parallel programming with bounded buffer producer consumer/problems, readers/writers problems and other "toy problems" solved using semaphores, followed by proof by induction that the solution is correct. And Linus did, eventually, finish a degree on computer science.