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

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.