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Spinlock

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Spinlock
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In software engineering, 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, or "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 durations, both preventing other threads from running and requiring re-scheduling. The longer a lock is held by a thread, the greater the risk that it will be interrupted by the O/S scheduler while holding the lock. If this happens, other threads will be left "spinning" (repeatedly trying to acquire the lock), while the thread holding the lock is not making progress towards releasing it. The result is a semi-deadlock until the thread holding the lock can finish and release it. This is especially true on a single-processor system, where each waiting thread of the same priority is likely to waste its quantum (allocated time where a thread can run) spinning until the thread that holds the lock is finally finished.