spinlock

spinlock is a lock where the thread simply waits in a loop("spins") repeatedly checking until the lock becomes available.
-->it's busy waiting
-->only likely to be locked for a short period of time.this avoid overhead from operating system process re-scheduling or context switching.
-->often in operating system kernels.

quantum(分配量)
are two alternatives that avoid this:

   1. Do not acquire the lock. In many situations it is possible to design data structures that do not require locking, e.g. by using per thread data, or by using per-cpu data and disabling interrupts.
   2. Switch to a different thread while waiting (sometimes called sleeplocks). This typically involves attaching the current thread to a queue of threads waiting for the lock, then switching to another one. This scheme also has the advantages that it guarantees that resource starvation does not occur as long as all threads eventually relinquish locks they acquire and scheduling decisions can be made about which thread should progress first.