It's just a lock. It's analogous to a pthread_mutex_t (which on Linux is a futex), where the operation in the uncontended case is a single atomic access to sha

It's just a lock. It's analogous to a pthread_mutex_t (which on Linux is a futex), where the operation in the uncontended case is a single atomic access to shared memory. The contended case needs to enter the kernel to suspend, or to make a decision about who to wake up, and that seems to be the subject of the blog post.