如果您要自动更改检查值，条件变量是否仍需要互斥锁? [英] Do condition variables still need a mutex if you're changing the checked value atomically?

问题描述

以下是使用条件变量的典型方法:

Here is the typical way to use a condition variable:

// The reader(s)
lock(some_mutex);
if(protected_by_mutex_var != desired_value)
    some_condition.wait(some_mutex);
unlock(some_mutex);

// The writer
lock(some_mutex);
protected_by_mutex_var = desired_value;
unlock(some_mutex);
some_condition.notify_all();

但是，如果通过比较和交换指令自动设置了protected_by_mutex_var，那么互斥锁是否有任何用途(除了pthread和其他API要求您传递互斥锁外)?它是用于执行条件的保护状态吗?如果不是，那么这样做安全吗?:

But if protected_by_mutex_var is set atomically by say, a compare-and-swap instruction, does the mutex serve any purpose (other than that pthreads and other APIs require you to pass in a mutex)? Is it protecting state used to implement the condition? If not, is it safe then to do this?:

// The writer
atomic_set(protected_by_mutex_var, desired_value);
some_condition.notify_all();

作者永远不会与读者的互斥体直接互动吗?请注意，'protected_by_mutex_var'名称不再是真正合适的名称(不再受互斥保护).如果是这样，甚至有必要让不同的读者使用相同的互斥锁吗?

With the writer never directly interacting with the reader's mutex? Note that the 'protected_by_mutex_var' name is no longer really appropriate (it's not mutex protected anymore). If so, is it even necessary that different readers use the same mutex?

推荐答案

想象一下以下情况:

| Thread 1                                            | Thread 2                                           |
| if(protected_by_mutex_var != desired_value) -> true |                                                    |
|                                                     | atomic_set(protected_by_mutex_var, desired_value); |
|                                                     | some_condition.notify_all();                       |
| some_condition.wait(some_mutex);                    |                                                    |

这种情况下，线程1正在等待可能永远不会到来的通知. 由于作用于条件的语句不属于变量read/atomic集合的一部分，因此这表示竞争条件.

This situation sees Thread 1 waiting for a notify that may never come. Because the statements acting on the condition are not part of the variable read / atomic set, this presents a race condition.

有效地使用互斥锁会使这些动作不可分割(假设对变量的所有访问均正常运行并锁定了互斥锁.)

Using the mutex effectively makes these actions inseparable (assuming all accesses to the variable behave properly and lock the mutex.)

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