[spacechild1]

Actually, many mutex implementations first spin in a loop and only then enter wait state. One prominent example is Windows' CRITICAL_SECTION (where the spin count is even configurable). However, I would never call it a spinlock. A mutex might contain a spinlock but not vice versa. After all, the defining property of a spinlock is that it never goes to sleep. One use case outside kernel programming is to protect shared resources in (soft) real time applications where the use of mutexes are forbidden (e.g. real time audio programming). Often you can use atomic variables or lockfree queues instead, but sometimes a spinlock is the only practical solution.