使用自旋锁的规则很简单:当且仅当锁持有的实际时间有界且足够小时使用自旋锁。

注意，通常用户实现的自旋锁不满足这个要求，因为它们不禁用中断。除非禁用了抢占，否则在自旋锁进行时进行抢占违反了有限时间要求。

足够小是一种判断，取决于上下文。

例外:一些内核编程即使在时间没有限制的情况下也必须使用自旋锁。特别是如果CPU没有工作要做，它别无选择，只能旋转，直到有更多的工作出现。

Special danger: in low level programming take great care when multiple interrupt priorities exist (usually there is at least one non-maskable interrupt). In this higher priority pre-emptions can run even if interrupts at the thread priority are disabled (such as priority hardware services, often related to the virtual memory management). Provided a strict priority separation is maintained, the condition for bounded real time must be relaxed and replaced with bounded system time at that priority level. Note in this case not only can the lock holder be pre-empted but the spinner can also be interrupted; this is generally not a problem because there's nothing you can do about it.

自旋锁和互斥锁同步机制现在非常常见。

让我们首先考虑Spinlock。

基本上，它是一个忙碌的等待操作，这意味着我们必须等待指定的锁被释放，然后才能继续进行下一个操作。概念上很简单，但实现起来却不是那么回事。例如:如果锁还没有释放，那么线程已经被换出并进入睡眠状态，我们应该处理它吗?当两个线程同时请求访问时，如何处理同步锁?

通常，最直观的想法是通过一个变量来处理同步，以保护临界区。互斥锁的概念是相似的，但它们仍然是不同的。重点关注:CPU利用率。自旋锁需要消耗CPU时间来等待执行操作，因此，我们可以总结两者之间的差异:

在同构多核环境中，如果在临界区花费的时间较小，则使用Spinlock，因为我们可以减少上下文切换时间。(单核比较不重要，因为有些系统在中间实现Spinlock开关)

在Windows中，使用Spinlock会将线程升级到DISPATCH_LEVEL，这在某些情况下可能是不允许的，所以这次我们必须使用互斥锁(APC_LEVEL)。

还请注意，在某些环境和条件下(例如在调度级别>= dispatch level的windows上运行)，您不能使用互斥，而应该使用自旋锁。 在unix上-同样的事情。

这是竞争对手stackexchange unix网站上的等效问题: https://unix.stackexchange.com/questions/5107/why-are-spin-locks-good-choices-in-linux-kernel-design-instead-of-something-more

windows系统上的调度信息: http://download.microsoft.com/download/e/b/a/eba1050f-a31d-436b-9281-92cdfeae4b45/IRQL_thread.doc

使用自旋锁的规则很简单:当且仅当锁持有的实际时间有界且足够小时使用自旋锁。

注意，通常用户实现的自旋锁不满足这个要求，因为它们不禁用中断。除非禁用了抢占，否则在自旋锁进行时进行抢占违反了有限时间要求。

足够小是一种判断，取决于上下文。

例外:一些内核编程即使在时间没有限制的情况下也必须使用自旋锁。特别是如果CPU没有工作要做，它别无选择，只能旋转，直到有更多的工作出现。

Special danger: in low level programming take great care when multiple interrupt priorities exist (usually there is at least one non-maskable interrupt). In this higher priority pre-emptions can run even if interrupts at the thread priority are disabled (such as priority hardware services, often related to the virtual memory management). Provided a strict priority separation is maintained, the condition for bounded real time must be relaxed and replaced with bounded system time at that priority level. Note in this case not only can the lock holder be pre-empted but the spinner can also be interrupted; this is generally not a problem because there's nothing you can do about it.

Spinlocks can actually perform very poorly on NUMA machines. The issue is easy to understand and very hard to fix (without switching to a mutex, that is). Consider a spinlock that lives in DRAM "near" core A, and threads on A and B contending for that lock. Assume that B is remote from this DRAM. As we all know, this means that memory accesses by A will be 5x or so faster than memory accesses by B, because B's accesses will need to traverse the bus of the NUMA chip, while A's accesses are local and hence avoid that bus traversal.

实际上，A的自旋逻辑将比B的快5倍或更多。是的，他们争论，B破坏了A，但影响是不对称的:当A赢得了下一次访问锁的竞争时，它将获得本地加载和存储，因此将以更高的指令速率旋转。当B旋转时，这些远程加载和存储将是缓慢的，所以B旋转是慢动作。

结果是，我们在Derecho的工作中观察到，我们得到了一个非常不公平的自旋。A比B更受青睐，被B锁定需要很长时间。

How would you observe this? In our case, we use LibFabrics, and that library has a few threads that get scattered over multiple cores. Within the LibFabric logic, A and B are spinning to lock and then check a completion queue associated with RDMA hardware. So the effect is that A gets to check this queue 5x more often than B. In cases where an action by B is needed (the completed operation at the head of that queue is owned by B), A effectively starves B for access -- slowing down LibFabrics in an extreme way, that snowballed to greatly impact our Derecho code. We've seen cases where A's access is so strongly favored that B might wait as long as 10ms for the lock -- even though under uncontended situations, B would grab this lock in 0.2us. So, the effect can be quite extreme.

结论?甚至不要考虑在NUMA系统上使用自旋锁，因为您的线程可能(1)在不同的NUMA内核上，(2)在分配自旋锁的DRAM上具有不同的位置。您将看到大量的性能问题!(3)当使用具有多个线程的第三方库时，请记住他们可能没有阅读此讨论，可能会出错!

梅基的回答非常准确。然而，在单个处理器上，当任务等待中断服务例程提供的锁时，使用自旋锁可能是有意义的。中断将控制权传递给ISR, ISR将准备好资源供等待任务使用。在将控制权交还给被中断的任务之前，它将以释放锁结束。旋转任务将发现自旋锁可用并继续。