semaphore mutex

Prior to that date, none of the interrupt-safe task synchronization and signaling mechanisms known to computer scientists was efficiently scalable for use by more than two tasks. Dijkstra's revolutionary, safe-and-scalable Semaphore was applied in bothcritical section protection andsignaling. And thus the confusion began.

http://www.barrgroup.com/Embedded-Systems/How-To/RTOS-Mutex-Semaphore

首先spinlock是只有在内核态才有的，当然你也可以在用户态自己实现，但是如果想要调用spinlock_t类型，那只有内核态才有。但是semaphore是内核态和用户态都有的，mutex是一种特殊的semaphore。

    spinlock是一种忙等待，也就是说，进程是不会睡眠的，只是一直在那里死循环。而mutex是睡等，也就是说，如果拿不到临界资源，那它会选择进程睡眠。那什么时候用spinlock，什么时候用mutex呢？首先，如果是在不允许睡眠的情况下，只能只用spinlock，比如中断的时候。然后如果临界区中执行代码的时间小于进程上下文切换的时间，那应该使用spinlock。反之应该使用mutex。

    那mutex和semaphore有什么区别呢？mutex是用作互斥的，而semaphore是用作同步的。也就是说，mutex的初始化一定是为1，而semaphore可以是任意的数，所以如果使用mutex，那第一个进入临界区的进程一定可以执行，而其他的进程必须等待。而semaphore则不一定，如果一开始初始化为0，则所有进程都必须等待。同时mutex和semaphore还有一个区别是，获得mutex的进程必须亲自释放它，而semaphore则可以一个进程获得，另一个进程释放。

Limiting Concurrent Access to Resources

Sometimes it is useful to allow more than one worker access to a resource at a time, while still limiting the overall number. For example, a connection pool might support a fixed number of simultaneous connections, or a network application might support a fixed number of concurrent downloads. A Semaphore is one way to manage those connections.

import logging
import random
import threading
import time

logging.basicConfig(level=logging.DEBUG,
                    format='%(asctime)s (%(threadName)-2s) %(message)s',
                    )

class ActivePool(object):
    def __init__(self):
        super(ActivePool, self).__init__()
        self.active = []
        self.lock = threading.Lock()
    def makeActive(self, name):
        with self.lock:
            self.active.append(name)
            logging.debug('Running: %s', self.active)
    def makeInactive(self, name):
        with self.lock:
            self.active.remove(name)
            logging.debug('Running: %s', self.active)

def worker(s, pool):
    logging.debug('Waiting to join the pool')
    with s:
        name = threading.currentThread().getName()
        pool.makeActive(name)
        time.sleep(0.1)
        pool.makeInactive(name)

pool = ActivePool()
s = threading.Semaphore(2)
for i in range(4):
    t = threading.Thread(target=worker, name=str(i), args=(s, pool))
    t.start()