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

From: Matthew Vogt (mvogt_at_[hidden])
Date: 2004-07-06 18:20:44

Michael Glassford <glassfordm <at> hotmail.com> writes:

> Translating this to the constructor model of promotion would have to 
> look something like this, I guess:
> 
> void f(read_write_mutex& m)
> {
>      read_write_mutex::try_read_lock r(m);
> 
>      //do "read stuff"
> 
>      read_write_mutex::try_write_lock w(r, /*maybe a parameter here to 
> use try_promote() instead of promote()*/);
> 
>      if (!w.locked())
>      {
>          r.unlock();
>          w.lock();
>          //redo some of "read stuff"
>      }
> 
>     //do write stuff
> }

You could encapsulate some of that in the constructor of the write lock:

void f(read_write_mutex& m)
{
  read_write_mutex::read_lock r(m);

  do_read_stuff();

  read_write_mutex::write_lock w(r);  // Upgrade, releasing and re-acquiring
                                      // if necessary
  if (!w.atomically_promoted())
  {
     do_read_stuff();
  }

  do_write_stuff();
}

It certainly looks error-prone, by requiring a test and the repetition of the
earlier code.  Makes you wonder if it should have a function to execute on
non-atomic upgrade:

{
  read_write_mutex::read_lock r(m);

  do_read_stuff();

  read_write_mutex::write_lock w(r, bind(this, &class::do_read_stuff));

  do_write_stuff();
}

You could reduce further, but I'm sure it will collapse under the weight of
real code :)

Matt