Yes, but the Linux kernel is the one I'm more familiar with. So I looked for another example: glibc's pthread_mutex_lock. The code is at nptl/pthread_mutex_lock

Yes, but the Linux kernel is the one I'm more familiar with. So I looked for another example: glibc's pthread_mutex_lock. The code is at nptl/pthread_mutex_lock.c and the "pause" instruction here is called atomic_spin_nop() (or BUSY_WAIT_NOP on older versions), defined by sysdeps/x86_64/atomic-machine.h. Yet again, it does a single "pause" instruction in the loop, and the loop condition tests the lock.