AutoResetEvent::Signal

[jurgen-kluft]

Hello, i have a question related to the signal function of AutoResetEvent where the load statement of m_status is not executed again after a fail of compare_exchange_weak.

void signal()  
{
    /// --> this below load statement of m_status should be in the for loop
    int oldStatus = m_status.load(std::memory_order_relaxed);
    for (;;)    // Increment m_status atomically via CAS loop.
    {
        assert(oldStatus <= 1);
        int newStatus = oldStatus < 1 ? oldStatus + 1 : 1;
        if (m_status.compare_exchange_weak(oldStatus, newStatus, std::memory_order_release, std::memory_order_relaxed))
            break;
        // The compare-exchange failed, likely because another thread changed m_status.
        // oldStatus has been updated. Retry the CAS loop.
        // --> We can retry the CAS loop but we still have an old 'oldStatus' so the next CAS
        // --> will surely fail again.
    }
    if (oldStatus < 0)
        m_sema.signal();    // Release one waiting thread.
}

[preshing]

If compare_exchange_weak fails, it automatically loads the latest value of m_status into oldStatus, which is passed by reference.

See here: "Otherwise, loads the actual value stored in *this into expected (performs load operation)."

[jurgen-kluft]

Thanks Jeff, learned something today :-)

On 14 November 2017 at 21:06, Jeff Preshing notifications@github.com wrote:

Closed #10 https://github.com/preshing/cpp11-on-multicore/issues/10.

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