如何自动更新最大值？ [英] How to atomically update a maximum value?

问题描述

在序列代码中，更新最大值可以简单地通过

In serial code, updating a maximum could be accomplished simply by

template<typename T>
void update_maximum(T& maximum_value, T const& value) noexcept
{
  if(value > maximum_value) maximum_value = value;
}

但是，对于 ; T> 含有最大值的变量：

However, how should this be done for an atomic<T> variable holding the maximum value:

template<typename T>
void update_maximum(std::atomic<T>& maximum_value, T const& value) noexcept
{
   // want a lock-free, thread-safe implementation
}

显然，来自串行版本的代码不起作用，因为另一个线程可能会改变 maximum_value 。可以使用 compare_exchange （它比较 == 而不是> ）来实现这个？如何？

Obviously, the code from the serial version doesn't work, as another thread may alter maximum_value between the load and the store. Can one use the compare_exchange (which compares == rather than >) to implement this? how?

请注意，不允许显式锁定（允许的唯一锁定是可能伴随 std :: atomic< T> ;

Note that explicit locks are not allowed (the only lock allowed is that which may come with the implementation of std::atomic<T>).

推荐答案

在单个操作中似乎不可能，可以做一个循环，尝试这样做，直到它终于成功或原子变量的值变得大于 value ：

It doesn't seem to be possible in a single operation, but you can make a loop, that tries to do this, until it finally succeeds or value in atomic variable becomes bigger than value:

template<typename T>
void update_maximum(std::atomic<T>& maximum_value, T const& value) noexcept
{
    T prev_value = maximum_value;
    while(prev_value < value &&
            !maximum_value.compare_exchange_weak(prev_value, value))
        ;
}

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