Re: [boost] [thread] countdown_latch

Subject: Re: [boost] [thread] countdown_latch
From: Vicente J. Botet Escriba (vicente.botet_at_[hidden])
Date: 2013-05-17 16:15:52

Le 14/05/13 08:16, Gaetano Mendola a écrit :
> On 14/05/2013 07.46, Vicente J. Botet Escriba wrote:
>> Le 14/05/13 00:44, Gaetano Mendola a écrit :
>>>
>>> Unlocking as soon as possible is something it should happen whenever
>>> is possible, but basically you are doing this kind of optimization:
>>>
>>> instead of
>>>
>>> void foo() {
>>>   boost::unique_lock<boost::mutex> lk(mutex_);
>>>   ....
>>>   return;
>>> }
>>>
>>> you are optimizing doing this:
>>>
>>> void foo() {
>>>   boost::unique_lock<boost::mutex> lk(mutex_);
>>>   ...
>>>   lk.unlock;
>>>   return;
>>> }
>>>
>>> and I'm asking, is it worth?  Do you have an evidence of it ?
>>> Having say that do whatever you believe is better in terms of clarity
>>> and correctness.
>>> I have seen also that in a couple of points the unique_lock can be
>>> replaced by a lock_guard (and possibly declared const).
>>>
>>>
>>>
>> Sorry Gaetano,
>>
>> I have an uncommitted version where
>>
>>      bool count_down(unique_lock<mutex> &lk)
>>      /// pre_condition (count_.value_ > 0)
>>      {
>>        BOOST_ASSERT(count_.value_ > 0);
>>        if (--count_.value_ == 0)
>>        {
>>          lk.unlock();
>>          count_.cond_.notify_all(); // unlocked !!!
>>          return true;
>>        }
>>        return false;
>>      }
>>
>> Maybe I'm wrong and I'm doing premature optimization and it is better to
>> use lock_guard and don't unlock before notifying.
>> As you point it is in any case clearer.
>
> Now it makes sense, unlocking prematurely an unique lock without the use
> of an extra scope is something not natural, it is at least a weird
> pattern, I believe the source of the "problem" is in the 
> implementation of that boost::detail::counter indeed as you can see it 
> has a counter
> and a condition and users of it are obliged to protect both counter and
> condition (even if not needed) look at inc_and_notify_all for example.
>
To which boost::detail::counter are you referring to?

Best,
Vicente