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

From: Peter Dimov (pdimov_at_[hidden])
Date: 2004-06-30 07:10:40

Michael Glassford wrote:
>
> Finally, to take two more realistic use cases:
>
>       void f(read_write_mutex m)
>       {
>           read_write_mutex::write_lock w(m);
>
>           //...
>
>           read_write_mutex::read_lock r(w); //lock promotion
>
>          //...
>       }
>
> And:
>
>       void f(read_write_mutex m)
>       {
>           read_write_mutex::write_lock r(m);
>
>           //...
>
>           read_write_mutex::read_lock w(r); //lock demotion
>
>          //...
>       }
>
> The second lock does unnecessary work in demoting/promoting the lock
> for the first lock, which then immediately unlocks it.

This is a very good point, since in the second example the unnecessary
promotion would block. This seems to suggest that read_write_lock is
necessary in such a scenario.