Re: [Boost-users] Does lock-less queue wait on empty queue ?

Subject: Re: [Boost-users] Does lock-less queue wait on empty queue ?
From: Nat Goodspeed (nat_at_[hidden])
Date: 2015-05-20 17:27:15

On Wed, May 20, 2015 at 5:15 PM, Gottlob Frege <gottlobfrege_at_[hidden]> wrote:

> Using condition variables does require mutexes, but you only need to
> grab the mutex when someone is actually waiting.  So you can have a
> separate (atomic) flag 'waiting' and then only grab the mutex when it
> is true.
>
> But that is an extra atomic op on every push (checking the flag).  Of
> course, it could probably be a relaxed atomic (I think), which is
> almost free on many platforms.
>
> And it gets tricky ensuring that you don't have a case where someone
> is waiting but every pusher missed seeing the flag.
>
> If the waiting is inside the queue implementation, you can possibly
> hide the flag inside the head pointer, getting the extra atomic op for
> free.
>
> I touched on some of this in my Lock-free talk at BoostCon/C++Now last week.

Aren't these tricky corner cases what "synchronic" is supposed to address?
http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2014/n4195.pdf