alt Hacker NewsThis is nonsense. If the lock hasn't been acquired, you don't spin to begin with and if the lock has been acquired and the lock is being released shortly after, the spinning avoids a context switch. If the maximum number of retries has been reached, the thread was going to sleep anyway and starts scheduling the next thread (which was only delayed by the few attempted spins). This means in the worst case the next spin will only happen once all the other queued up threads have had their turn and that's assuming you're immediately running into another acquired lock.
It's makes the worse case sufficiently bad and unfair such that it makes things worse overall. If the lock is contended by a thread with higher priority, then that blocking thread will have its priority increased. Now if the ends thread to get the lock is one spinning on it rather than actual high priority one, then this will repeat, leading to large latency on front of the high priority thread and a lot of misaligned CPU utilization by a lower priority thread.
Spinning on a CAS is far more expensive than spinning on most other instructions as well as it affects all core that may try to access that cache line, which may include things other than the lock itself.
Also consider how the system acts under high CPU load. You will end up with threads holding locks when not running leading to the majority of the time you miss the lock you spin all 100 times. This just exacerbate the CPU load issues even more. Hybrid locks are only helpful under lower CPU load.