Comment by kazinator

Before we had futexes in the Linux kernel, spinlocks were used to boostrap the implementation of everything else in the user space threading library.

If you have futexes you can try to grab a lock with an atomic operation and if that fails, go wait on the futex via system call, so there is no need to spin. Spinlocks then remain useful as an optimization, because there are situations in which it is cheaper to spin around a bunch of times until the thread on another processor gives up the lock, than to take a trip into the kernel.

You can also spin, but with a scheduler yield in the loop; we don't normally think of that as a spinlock. That's what you fall back on after spinning some number of times and failing to get the lock.

In the Linux kernel, spinlocks are the low level primitive. They are very efficient because unlike user space threading, they are not faced with guesswork about scheduling. They are "surgical".