conditional variables（下列内容摘自FreeBSD Device Drivers）：
Condition variables synchronize the execution of two or more threads based upon the value of an object. In contrast, locks synchronize threads by controlling their access to objects.
Condition variables are used in conjunction with locks to “block” threads until a condition is true. It works like this: A thread first acquires the foo lock. Then it examines the condition. If the condition is false, it sleeps on the bar condition variable. While asleep on bar , threads relinquish foo . A thread that causes the condition to be true wakes up the threads sleeping on bar . Threads woken up in this manner reacquire foo before proceeding.
lock的规则（下列内容摘自FreeBSD Kernel Developer’s Manual）：
The lock argument is a pointer to either mutex(9), rwlock(9), or sx(9) lock. A mutex(9) argument must be initialized with MTX_DEF and not MTX_SPIN. A thread must hold lock before calling cvwait(), cvwaitsig(), cvwaitunlock(), cvtimedwait(), or cvtimedwaitsig(). When a thread waits on a condition, lock is atomically released before the thread is blocked, then reacquired before the function call returns. In addition, the thread will fully drop the Giant mutex (even if recursed) while the it is suspended and will reacquire the Giant mutex before the function returns. The cvwaitunlock() function does not reacquire the lock before returning. Note that the Giant mutex may be specified as lock. However, Giant may not be used as lock for the cvwaitunlock() function. All waiters must pass the same lock in con- junction with cvp.