Re: [boost] [thread] countdown_latch

Subject: Re: [boost] [thread] countdown_latch
From: Vicente J. Botet Escriba (vicente.botet_at_[hidden])
Date: 2013-05-11 19:03:10

Le 11/05/13 23:25, Gaetano Mendola a écrit :
> On 27/04/2013 09.05, Vicente J. Botet Escriba wrote:
>> The change set is https://svn.boost.org/trac/boost/changeset/84055. To
>> see the sources, you would need to update the trunk or to take a look at
>> https://svn.boost.org/svn/boost/trunk/boost/thread/
>>
>> Please let me know what do you think.
>
> Is the explicit lk.unlock() inside the
>
> bool count_down(unique_lock<mutex> &lk);
>
> needed ?
>

> I see the two places where it's called are:
>
> void count_down()
> {
>   boost::unique_lock<boost::mutex> lk(mutex_);
>   count_down(lk);
> }
>
> and
>
> void count_down_and_wait()
> {
>   boost::unique_lock<boost::mutex> lk(mutex_);
>   if (count_down(lk))
>   {
>     return;
>   }
>   count_.cond_.wait(lk, detail::counter_is_zero(count_));
> }
>
> in both cases the unique_lock does the job, or I'm missing
> something?
Not really. It is an optimization. The mutex don't need to be lock when 
the condition is notified.
>
> Also I would add a new method to boost::detail::counter
>
>   bool dec_and_notify_all_if_value(const std::size a_value)
>   {
>     if (--value_ == a_value)
>     {
>       cond_.notify_all();
>       return true;
>     }
>     return false;
>   }
>
>
> This way you can further simplify the count_down(unique_lock<mutex> &lk)
> method:
>
>     //lk is here only to assure it's a thread safe call
>     bool count_down(unique_lock<mutex> &lk)
>     /// pre_condition (count_.value_ > 0)
>     {
>       BOOST_ASSERT(count_.value_ > 0);
>       return count_.dec_and_notify_all_if_value(0);
>     }
When this function would be used?

Best,
Vicente