Atomic

一些使用C++ atomic library的code。

chromium/base/atomic_ref_count.h

// This is a low level implementation of atomic semantics for reference
// counting.  Please use base/memory/ref_counted.h directly instead.

#ifndef BASE_ATOMIC_REF_COUNT_H_
#define BASE_ATOMIC_REF_COUNT_H_

#include <atomic>

namespace base
{

class AtomicRefCount
{
public:
    constexpr AtomicRefCount() :
                    ref_count_(0)
    {
    }
    explicit constexpr AtomicRefCount(int initial_value) :
                    ref_count_(initial_value)
    {
    }

    // Increment a reference count.
    // Returns the previous value of the count.
    int Increment()
    {
        return Increment(1);
    }

    // Increment a reference count by "increment", which must exceed 0.
    // Returns the previous value of the count.
    int Increment(int increment)
    {
        return ref_count_.fetch_add(increment, std::memory_order_relaxed);
    }

    // Decrement a reference count, and return whether the result is non-zero.
    // Insert barriers to ensure that state written before the reference count
    // became zero will be visible to a thread that has just made the count zero.
    bool Decrement()
    {
        // TODO(jbroman): Technically this doesn't need to be an acquire operation
        // unless the result is 1 (i.e., the ref count did indeed reach zero).
        // However, there are toolchain issues that make that not work as well at
        // present (notably TSAN doesn't like it).
        return ref_count_.fetch_sub(1, std::memory_order_acq_rel) != 1;
    }

    // Return whether the reference count is one.  If the reference count is used
    // in the conventional way, a refrerence count of 1 implies that the current
    // thread owns the reference and no other thread shares it.  This call
    // performs the test for a reference count of one, and performs the memory
    // barrier needed for the owning thread to act on the object, knowing that it
    // has exclusive access to the object.
    bool IsOne() const
    {
        return ref_count_.load(std::memory_order_acquire) == 1;
    }

    // Return whether the reference count is zero.  With conventional object
    // referencing counting, the object will be destroyed, so the reference count
    // should never be zero.  Hence this is generally used for a debug check.
    bool IsZero() const
    {
        return ref_count_.load(std::memory_order_acquire) == 0;
    }

    // Returns the current reference count (with no barriers). This is subtle, and
    // should be used only for debugging.
    int SubtleRefCountForDebug() const
    {
        return ref_count_.load(std::memory_order_relaxed);
    }

private:
    std::atomic_int ref_count_;
};

}  // namespace base

#endif  // BASE_ATOMIC_REF_COUNT_H_

Decrement

    // Decrement a reference count, and return whether the result is non-zero.
    // Insert barriers to ensure that state written before the reference count
    // became zero will be visible to a thread that has just made the count zero.
    bool Decrement()
    {
        // TODO(jbroman): Technically this doesn't need to be an acquire operation
        // unless the result is 1 (i.e., the ref count did indeed reach zero).
        // However, there are toolchain issues that make that not work as well at
        // present (notably TSAN doesn't like it).
        return ref_count_.fetch_sub(1, std::memory_order_acq_rel) != 1;
    }

注释的表明意思是:

Insert barriers的目的是保证在reference count成为0之前，被写入的state(state written before the reference count became zero)对另外一个刚刚使得reference count成为0的thread是可见的(visible)

它的含义是什么呢?