二进制信号量和互斥量之间有区别吗?或者它们本质上是相同的?


当前回答

在窗口,差异如下所示。 MUTEX:成功执行等待的进程必须执行一个信号,反之亦然。二进制信号量:不同的进程可以在一个信号量上执行等待或信号操作。

其他回答

答案可能取决于目标操作系统。例如,我所熟悉的至少一个RTOS实现允许对单个OS互斥量进行多个连续的“get”操作,只要它们都来自同一个线程上下文中。在允许另一个线程获得互斥量之前,多个get必须被相等数量的put替换。这与二进制信号量不同,对于二进制信号量,无论线程上下文如何,一次只允许一个get。

这种互斥锁背后的思想是,通过一次只允许一个上下文修改数据来保护对象。即使线程获得了互斥量,然后调用进一步修改对象的函数(并在自己的操作周围获得/放置保护互斥量),这些操作仍然应该是安全的,因为它们都发生在单个线程下。

{
    mutexGet();  // Other threads can no longer get the mutex.

    // Make changes to the protected object.
    // ...

    objectModify();  // Also gets/puts the mutex.  Only allowed from this thread context.

    // Make more changes to the protected object.
    // ...

    mutexPut();  // Finally allows other threads to get the mutex.
}

当然,在使用此特性时,必须确保单个线程中的所有访问都是安全的!

我不确定这种方法有多普遍,或者它是否适用于我所熟悉的系统之外。有关这种互斥锁的示例,请参阅ThreadX RTOS。

关于这个主题的好文章:

互斥量与信号量——第1部分:信号量 互斥量与信号量——第2部分:互斥量 互斥量与信号量——第3部分(最后一部分):互斥问题

来自第二部分:

The mutex is similar to the principles of the binary semaphore with one significant difference: the principle of ownership. Ownership is the simple concept that when a task locks (acquires) a mutex only it can unlock (release) it. If a task tries to unlock a mutex it hasn’t locked (thus doesn’t own) then an error condition is encountered and, most importantly, the mutex is not unlocked. If the mutual exclusion object doesn't have ownership then, irrelevant of what it is called, it is not a mutex.

在理论层面上,它们在语义上并无不同。您可以使用信号量实现互斥量,反之亦然(参见这里的示例)。在实践中,实现是不同的,它们提供的服务也略有不同。

实际的区别(就围绕它们的系统服务而言)在于互斥锁的实现旨在成为一种更轻量级的同步机制。在oracle语言中,互斥锁被称为锁存器,而信号量被称为等待。

在最低级别,他们使用某种原子测试和设置机制。它读取内存位置的当前值,计算某种条件,并在一条不能中断的指令中写入该位置的值。这意味着您可以获得一个互斥锁,并测试是否有人在您之前拥有它。

典型的互斥量实现有一个进程或线程执行test-and-set指令,并评估是否有其他东西设置了互斥量。这里的关键点是与调度程序没有交互,因此我们不知道(也不关心)谁设置了锁。然后,我们要么放弃我们的时间片,并在任务重新调度时再次尝试它,要么执行自旋锁。自旋锁是这样一种算法:

Count down from 5000:
     i. Execute the test-and-set instruction
    ii. If the mutex is clear, we have acquired it in the previous instruction 
        so we can exit the loop
   iii. When we get to zero, give up our time slice.

当我们完成执行受保护的代码(称为临界区)时,我们只需将互斥量的值设置为零或其他表示“清除”的值。如果有多个任务试图获取互斥量,那么下一个计划在互斥量释放后的任务将获得对资源的访问权。通常情况下,您可以使用互斥来控制同步资源,在这种资源中,只需要在很短的时间内对其进行独占访问,通常是对共享数据结构进行更新。

A semaphore is a synchronised data structure (typically using a mutex) that has a count and some system call wrappers that interact with the scheduler in a bit more depth than the mutex libraries would. Semaphores are incremented and decremented and used to block tasks until something else is ready. See Producer/Consumer Problem for a simple example of this. Semaphores are initialised to some value - a binary semaphore is just a special case where the semaphore is initialised to 1. Posting to a semaphore has the effect of waking up a waiting process.

一个基本的信号量算法如下所示:

(somewhere in the program startup)
Initialise the semaphore to its start-up value.

Acquiring a semaphore
   i. (synchronised) Attempt to decrement the semaphore value
  ii. If the value would be less than zero, put the task on the tail of the list of tasks waiting on the semaphore and give up the time slice.

Posting a semaphore
   i. (synchronised) Increment the semaphore value
  ii. If the value is greater or equal to the amount requested in the post at the front of the queue, take that task off the queue and make it runnable.  
 iii. Repeat (ii) for all tasks until the posted value is exhausted or there are no more tasks waiting.

在二进制信号量的情况下,两者之间的主要实际区别是围绕实际数据结构的系统服务的性质。

编辑:正如evan正确地指出的那样,自旋锁会降低单个处理器的速度。你只能在多处理器上使用自旋锁,因为在单处理器上,持有互斥锁的进程永远不会在另一个任务运行时重置它。自旋锁只在多处理器架构上有用。

http://www.geeksforgeeks.org/archives/9102将详细讨论。

互斥是一种锁机制,用于同步对资源的访问。 信号量是一种信号机制。

如果他/她想使用二进制信号量来代替互斥量,这取决于程序员。

Mutex is used to protect the sensitive code and data, semaphore is used to synchronization.You also can have practical use with protect the sensitive code, but there might be a risk that release the protection by the other thread by operation V.So The main difference between bi-semaphore and mutex is the ownership.For instance by toilet , Mutex is like that one can enter the toilet and lock the door, no one else can enter until the man get out, bi-semaphore is like that one can enter the toilet and lock the door, but someone else could enter by asking the administrator to open the door, it's ridiculous.