Re: [boost] Re: Boost.Threads: Do we need all those mutexes?

From: Peter Dimov (pdimov_at_[hidden])
Date: 2004-07-12 12:31:52

Michael Glassford wrote:
> Peter Dimov wrote:
>
>> It seems wrong (and the implementation seems buggy), but
>> as usual, I may be missing something.
>
> Alexander Terekhov has mentioned one problem with the Win32 condition
> variable implementation that I recall (I have the link somewhere but
> don't see it at the moment); do you have others in mind?

For a condition being used with a recursive_mutex, condition::wait does:

        typename lock_ops::lock_state state;
        lock_ops::unlock(mutex, state);
        m_impl.do_wait(...);
        lock_ops::lock(mutex, state);

The (Windows, for example) recursive mutex stores its lock count in state:

void recursive_mutex::do_unlock(cv_state& state)
{
    state = m_count;
    m_count = 0;

    if (m_critical_section)
        release_critical_section(m_mutex);
    else
        release_mutex(m_mutex);
}

and restores it in do_lock( cv_state& ).

However, if there is a thread switch just after m_count = 0, and then the
recursive_mutex is locked, its m_count would now be 1. After the do_wait,
do_lock( cv_state& ) will restore the m_count to the original value, and the
count update caused by the "extra" lock would be lost. Oder?