[英]Passing a dummy lock to std::condition_variable_any::wait
Suppose there are three threads A, B, and C. B and C suspend at a certain point, waiting for A to signal them to continue. 假设有三个线程A,B和C.B和C在某一点暂停,等待A发信号通知它们继续。 Among the thread synchronization facilities provided by standard C++,
std::condition_variable
seems to best fit in here (though still bad). 在标准C ++提供的线程同步工具中,
std::condition_variable
似乎最适合这里(尽管仍然很糟糕)。 Since std::condition_variable
must be used with a lock, the code for B and C may contain lines like: 由于
std::condition_variable
必须与锁一起使用,因此B和C的代码可能包含以下行:
{
std::mutex mut;
std::unique_lock<std::mutex> lock(mut);
cond_var.wait(lock); // cond_var is a global variable of type std::condition_variable`
}
Note that mut
is used here not for synchronization purposes at all, but just to fit the signature of std::condition_variable::wait
. 注意,
mut
在这里根本不用于同步目的,只是为了适应std::condition_variable::wait
的签名。 With this observation, I'm thinking that maybe we can do better by implementing a dummy lock class, let's say dummy_lock
, and replace std::condition_variable
with std::condition_variable_any
. 有了这个观察,我想也许我们可以通过实现一个虚拟锁类来
dummy_lock
,比如说dummy_lock
,并用std::condition_variable
替换std::condition_variable_any
。 dummy_lock
meets the BasicLockable requirements with all its methods essentially doing nothing. dummy_lock
满足BasicLockable要求,其所有方法基本上什么都不做。 Thereby, we get code similar to the following: 因此,我们得到类似于以下代码:
{
dummy_lock lock;
cond_var.wait(lock); // cond_var is a global variable of type std::condition_variable_any`
}
This, if works at all, should be of higher efficiency than the original one. 如果可以的话,这应该比原来的效率更高。 But the question is, does it even work according to the standard (language-lawyers are apt here) ?
但问题是,它是否按照标准运作(语言律师在这里适用) ? Even if it works, this is by-no-means an elegant solution.
即使它有效,这绝不是一个优雅的解决方案。 So, do any of you folks have better ideas?
那么,你们中的任何人都有更好的想法吗?
You are working on a false premise. 你正在做一个错误的前提。
The mutex does not only protect the condition predicate, it also protects the condition_variable itself. 互斥锁不仅可以保护条件谓词,还可以保护condition_variable本身。
So the mutex should be at the same scope as the condition_variable and all locks should lock that same mutex. 因此,互斥锁应与condition_variable在同一范围内,并且所有锁应锁定相同的互斥锁。
like this: 像这样:
// global scope
std::mutex mut;
std::condition_variable cond_var;
// thread scope
{
std::unique_lock<std::mutex> lock(mut);
cond_var.wait(lock);
}
see here: Why do pthreads' condition variable functions require a mutex? 看到这里: 为什么pthreads的条件变量函数需要一个互斥量?
What you're suggesting would be undefined behavior. 你所建议的是未定义的行为。 The section on
condition_variable_any
leads with, emphasis mine: condition_variable_any
上的部分引导着,强调我的:
A
Lock
type shall meet theBasicLockable
requirements (30.2.5.2).Lock
类型应满足BasicLockable
要求(30.2.5.2)。 [ Note: All of the standard mutex types meet this requirement.[注意:所有标准互斥锁类型都符合此要求。 If a Lock type other than one of the standard mutex types or a unique_lock wrapper for a standard mutex type is used with condition_variable_any, the user must ensure that any necessary synchronization is in place with respect to the predicate associated with the
condition_variable_any
instance.如果标准互斥锁类型之一以外的锁定类型或标准互斥锁类型的unique_lock包装器与condition_variable_any一起使用, 则用户必须确保对与
condition_variable_any
实例关联的谓词进行任何必要的同步。 —end note ]- 尾注]
The BasicLockable
requirements themselves don't just describe the interface, they also describe the required semantics: BasicLockable
需求本身不只是描述接口,它们还描述了所需的语义:
A type
L
meets theBasicLockable
requirements if the following expressions are well-formed and have the specified semantics (m denotes a value of type L).如果以下表达式格式正确并且具有指定的语义 (m表示类型L的值),则类型
L
满足BasicLockable
要求。
m.lock()
2 Effects: Blocks until a lock can be acquired for the current execution agent.2效果:阻止直到可以为当前执行代理获取锁定。 If an exception is thrown then a lock shall not have been acquired for the current execution agent.
如果抛出异常,则不应为当前执行代理获取锁。
If your dummy_lock
doesn't actually acquire a lock, it isn't a BasicLockable
, and so you fail to meet the premise for condition_variable_any
. 如果你的
dummy_lock
实际上没有获得锁,那么它不是BasicLockable
,因此你无法满足condition_variable_any
的前提。 At that point, all bets are off, and you cannot expect wait()
to do anything reasonable. 此时,所有赌注都已关闭,您不能指望
wait()
做任何合理的事情。
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