Spinlock

spin lock: busy waiting
semaphore: sleep and wake-up
spin lock 适用于切换快的场合, 否则会浪费大量cpu时间

a spinlock is a lock where the thread simply waits in a loop ("spins") repeatedly checking until the lock becomes available. As the thread remains active but isn't performing a useful task, the use of such a lock is a kind of busy waiting. Once acquired, spinlocks will usually be held until they are explicitly released, although in some implementations they may be automatically released if the thread blocks (aka "goes to sleep").

Spinlocks are efficient if threads are only likely to be blocked for a short period of time, as they avoid overhead from operating system process re-scheduling or context switching. For this reason, spinlocks are often used inside operating system kernels. However, spinlocks become wasteful if held for longer, both preventing other threads from running and requiring re-scheduling. The longer you hold the lock, the greater the risk that you will be interrupted by the O/S scheduler while holding it. If this happens, other threads will be left spinning on the lock, despite the fact that you are not making progress towards releasing it. This is especially true on a single-processor system, where each waiting thread of the same priority is likely to waste its full quantum spinning until the thread that holds the lock is finally re-scheduled.

Implementing spinlocks is difficult, because one must take account of the possibility of simultaneous access to the lock to prevent race conditions. Generally this is only possible with special assembly language instructions, such as atomic test-and-set operations, and cannot be implemented from high level languages like C.[1] On architectures without such operations, or if high-level language implementation is required, a non-atomic locking algorithm may be used, e.g. Peterson's algorithm. But note that such an implementation may require more memory than a spinlock, be slower to allow progress after unlocking, and may not be implementable in a high-level language if out-of-order execution is in use.