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That looks like some API/kernel call overhead; you've moved the fast path of the semaphore implementation into user space. But what you have there is undeniably a semaphore implementation: atomically tweak a counter, and based on that result, wait or signal.

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> you've moved the fast path of the semaphore implementation into user space.

I see now why your original comment was a bit inflammatory. I should have been more clear in the post that by "lightweight", I meant exactly that: "fast path in user space". I guess not everyone shares this vocabulary. I'll improve the post.

You're right that this lightweight mutex is a semaphore, of course. But not every semaphore is a lightweight mutex. So the technique isn't pointless.

Seems like that's what any sane modern user space sync primitives would do: do the atomic ops in user space for the fast path, enter the kernel when it's time to block. Like the "futex" in Linux.

I suppose it is a fair point that the thing you use to enter the kernel doesn't have to be itself a semaphore.

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