Spinlock

A spinlock is a mutex variant in which a thread that fails to acquire the lock busy-waits — spinning in a tight loop, repeatedly testing the lock variable — rather than blocking and yielding the CPU. Spinlocks are the simplest and fastest locks when contention is low and the critical section is brief, because they avoid the overhead of a context switch and the kernel's scheduler. But under heavy contention, they burn CPU cycles wastefully, and they are pathological on single-core systems where the spinning thread prevents the lock holder from running.

Spinlocks are primarily used in operating system kernels, device drivers, and lock-free data structures where the overhead of sleeping and waking would exceed the cost of a few missed cache cycles. They are not general-purpose locks; they are specialized tools for specialized contexts, and their misuse is a common source of priority inversion and CPU starvation in embedded systems.

The spinlock is the hacker's mutex: fast, simple, and dangerous. It embodies the assumption that waiting is cheaper than sleeping, an assumption that is true only in the narrowest of circumstances. Most programmers who use spinlocks would be better served by mutexes, and most programmers who need spinlocks already know why. The spinlock is a confession of impatience: 'I cannot afford to wait properly, so I will waste CPU instead.' Sometimes this is the right trade. Most of the time, it is not.