单个位的 spinlock 实现 folly::PicoSpinLock

之前提到单字节且为 POD 结构的自旋锁实现folly::MicroSpinLock，而folly:PicoSpinLock则只需要一个位！实现代码在https://github.com/facebook/folly/blob/master/folly/synchronization/PicoSpinLock.h。

其实现更需要 CPU 底层指令的支持。具体不多说。想说的一个有意思的地方在于，实现文件的最前面提出：

N.B. You most likely do not want to use PicoSpinLock or any other kind of spinlock. Consider MicroLock instead.

In short, spinlocks in preemptive multi-tasking operating systems have serious problems and fast mutexes like std::mutex are almost certainly the better choice, because letting the OS scheduler put a thread to sleep is better for system responsiveness and throughput than wasting a timeslice repeatedly querying a lock held by a thread that's blocked, and you can't prevent userspace programs blocking.

Spinlocks in an operating system kernel make much more sense than they do in userspace.

这里说的应该跟内核里使用自旋锁会关闭中断有关。在用户态里使用自旋锁，无法关闭中断的情况下，一旦被中断切走，自旋锁就真的在空空干等了。