Linux下互斥量与条件变量详细解析

忘是亡心i 2022-04-16 05:25 204阅读 0赞

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**1. 首先pthread\_cond\_wait 的定义是这样的**

***The pthread\_cond\_wait()* and *pthread\_cond\_timedwait()* functions are used to block on a condition variable. They are called with *mutex* locked by the calling thread or undefined behaviour will result.**

**These functions atomically release *mutex* and cause the calling thread to block on the condition variable *cond*;atomically here means "atomically with respect to access by anotherthread to the mutex and then the condition variable". That is, ifanother thread is able to acquire the mutex after the about-to-blockthread has released it, then a subsequent call to*[pthread\_cond\_signal() ][pthread_cond_signal_]*or *[pthread\_cond\_broadcast() ][pthread_cond_broadcast_]*in that thread behaves as if it were issued after the about-to-block thread has blocked.**

**2. 由上解释可以看出,pthread\_cond\_wait() 必须与pthread\_mutex 配套使用。(wait的内部操作：一进入wait状态就unclock，在wait结束前lock)**

**pthread\_cond\_wait()函数一进入wait状态就会自动release mutex.**

**In Thread1:**

**pthread\_mutex\_lock(&m\_mutex);  
pthread\_cond\_wait(&m\_cond,&m\_mutex);  
pthread\_mutex\_unlock(&m\_mutex);**

**In Thread2:**

**pthread\_mutex\_lock(&m\_mutex);  
pthread\_cond\_signal(&m\_cond);  
pthread\_mutex\_unlock(&m\_mutex);**

**为什么要与pthread\_mutex 一起使用呢？这是为了应对线程1在调用pthread\_cond\_wait()但线程1还没有进入wait cond的状态的时候，此时线程2调用了cond\_singal 的情况。 如果不用mutex锁的话，这个cond\_singal就丢失了。加了锁的情况是，线程2必须等到 mutex被释放（也就是 pthread\_cod\_wait() 进入wait\_cond状态 并自动释放mutex） 的时候才能调用cond\_singal(前提：线程2也使用mutex)。**

**3. pthread\_cond\_wait() 一旦wait成功获得cond 条件的时候会自动 lock mutex.**

**这就会出现另一个问题。这是因为**

***The pthread\_cond\_wait()* and *pthread\_cond\_timedwait()* is a cancellation point.**

**In Thread3:**

**pthread\_cancel(&m\_thread);**

**pthread\_join();**

**因为*pthread\_cond\_wait()* and *pthread\_cond\_timedwait()* 是线程退出点函数，因此在Thread3中**

**可以调用pthread\_cancel()来退出线程1。那样显然线程1会在pthread\_cond\_wait(&m\_cond,&m\_mutex); 和pthread\_mutex\_unlock(&m\_mutex); 之间退出， pthread\_cond\_wait()函数返回后自动lock住了mutex,这个时候线程1退出（并没有运行到pthread\_mutex\_unlock（）），如果Thread2这个时候就再也得不到lock状态了。**

**通常解决这个问题的办法如下**

**void cleanup(void \*arg)  
\{  
pthread\_mutex\_unlock(&mutex);  
\}  
void\* thread1(void\* arg)  
\{  
pthread\_cleanup\_push(cleanup, NULL); *// thread cleanup handler*  
pthread\_mutex\_lock(&mutex);  
pthread\_cond\_wait(&cond, &mutex);  
pthread\_mutex\_unlock(&mutex);  
pthread\_cleanup\_pop(0);  
\}**

**该方法也可用于其它可能异常终止或退出的线程。**

**LINUX环境下多线程编程肯定会遇到需要条件变量的情况，此时必然要使用pthread\_cond\_wait()函数。但这个函数的执行过程比较难于理解。  
pthread\_cond\_wait()的工作流程如下（以MAN中的EXAMPLE为例）：  
Consider two shared variables x and y, protected by the mutex mut, and a condition vari-  
able cond that is to be signaled whenever x becomes greater than y.**

**int x,y;  
pthread\_mutex\_t mut = PTHREAD\_MUTEX\_INITIALIZER;  
pthread\_cond\_t cond = PTHREAD\_COND\_INITIALIZER;**

**Waiting until x is greater than y is performed as follows:**

**pthread\_mutex\_lock(&mut);  
*while (x <= y)* \{  
pthread\_cond\_wait(&cond, &mut);  
\}  
*/\* operate on x and y \*/*  
pthread\_mutex\_unlock(&mut);**

**Modifications on x and y that may cause x to become greater than y should signal the con-  
dition if needed:**

**pthread\_mutex\_lock(&mut);  
/\* modify x and y \*/  
if (x > y) pthread\_cond\_broadcast(&cond);  
pthread\_mutex\_unlock(&mut);**

**这个例子的意思是，两个线程要修改X和Y的值，第一个线程当X<=Y时就挂起，直到X>Y时才继续执行（由第二个线程可能会修改X,Y的值，当X>Y时唤醒第一个线程），即首先初始化一个普通互斥量mut和一个条件变量cond。之后分别在两个线程中分别执行如下函数体：**

**pthread\_mutex\_lock(&mut);  
while (x <= y) \{  
pthread\_cond\_wait(&cond, &mut);  
\}  
/\* operate on x and y \*/  
pthread\_mutex\_unlock(&mut);**

**和： pthread\_mutex\_lock(&mut);  
/\* modify x and y \*/  
if (x > y) pthread\_cond\_signal(&cond);  
pthread\_mutex\_unlock(&mut);  
其实函数的执行过程非常简单，在第一个线程执行到pthread\_cond\_wait(&cond,&mut)时，此时如果X<=Y，则此函数就将mut互斥量解锁，再将cond条件变量加锁，此时第一个线程挂起（不占用任何CPU周期）。  
而在第二个线程中，本来因为mut被第一个线程锁住而阻塞，此时因为mut已经释放，所以可以获得锁mut，并且进行修改X和Y的值，在修改之后，一个IF语句判定是不是X>Y，如果是，则此时pthread\_cond\_signal()函数会唤醒第一个线程，并在下一句中释放互斥量mut。然后第一个线程开始从pthread\_cond\_wait()执行，首先要再次锁mut， 如果锁成功，再进行条件的判断（至于为什么用WHILE，即在被唤醒之后还要再判断，后面有原因分析），如果满足条件，则被唤醒进行处理，最后释放互斥量mut。**

**至于为什么在被唤醒之后还要再次进行条件判断（即为什么要使用while循环来判断条件），是因为可能有“惊群效应”。有人觉得此处既然是被唤醒的，肯定是满足条件了，其实不然。如果是多个线程都在等待这个条件，而同时只能有一个线程进行处理，此时就必须要再次条件判断，以使只有一个线程进入临界区处理。对此，转来一段：**

**引用下POSIX的RATIONALE：  
  
Condition Wait Semantics  
  
It is important to note that when pthread\_cond\_wait() andpthread\_cond\_timedwait() return without error, the associated predicatemay still be false. Similarly, when pthread\_cond\_timedwait() returnswith the timeout error, the associated predicate may be true due to anunavoidable race between the expiration of the timeout and thepredicate state change.  
  
The application needs to recheck the predicate on any return because itcannot be sure there is another thread waiting on the thread to handlethe signal, and if there is not then the signal is lost. The burden ison the application to check the predicate.  
  
Some implementations, particularly on a multi-processor, may sometimescause multiple threads to wake up when the condition variable issignaled simultaneously on different processors.  
  
In general, whenever a condition wait returns, the thread has tore-evaluate the predicate associated with the condition wait todetermine whether it can safely proceed, should wait again, or shoulddeclare a timeout. A return from the wait does not imply that theassociated predicate is either true or false.  
  
It is thus recommended that a condition wait be enclosed in the equivalent of a "while loop" that checks the predicate.  
  
从上文可以看出：  
1，pthread\_cond\_signal在多处理器上可能同时唤醒多个线程，当你只能让一个线程处理某个任务时，其它被唤醒的线程就需要继续 wait,while循环的意义就体现在这里了，而且规范要求pthread\_cond\_signal至少唤醒一个pthread\_cond\_wait上的线程，其实有些实现为了简单在单处理器上也会唤醒多个线程.  
2，某些应用，如线程池，pthread\_cond\_broadcast唤醒全部线程，但我们通常只需要一部分线程去做执行任务，所以其它的线程需要继续wait.所以强烈推荐此处使用while循环.**

**其实说白了很简单，就是pthread\_cond\_signal()也可能唤醒多个线程，而如果你同时只允许一个线程访问的话，就必须要使用while来进行条件判断，以保证临界区内只有一个线程在处理。**

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[http_blog.csdn.net_jiangjunshow]: https://blog.csdn.net/jiangjunshow/article/details/77338485
[pthread_cond_signal_]: http://www.opengroup.org/on%3Cwbr%3Elinepubs/007908775/xsh/pthread_cond_signal.html
[pthread_cond_broadcast_]: http://www.opengroup.org/on%3Cwbr%3Elinepubs/007908775/xsh/pthread_cond_broadcast.html
[SouthEast]: /images/20220416/0acc8e8b5be04bca8dd19565b23ee36d.png