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

From: Peter Dimov (pdimov_at_[hidden])
Date: 2004-07-06 05:01:48

Howard Hinnant wrote:
>
> Oops, that should be:
>
> vs this:
>
> void do_sale(rw_mutex& m)
> {
>    read_lock rl(m);
>
>    long balance = get_balance();
>    long cost = get_total_cost();
>
>    if (balance > cost)
>    {
>      rl.unlock(m);
>      write_lock wl(m);
>      cost = get_total_cost();
>      set_balance(get_balance() - cost);
>      wl.transfer_to_read_lock(rl);
>      balance = get_balance();
>    }
>    // ...
> }
>
> but still looks much simpler, and smaller code size to me.

The read lock is pointless in this case. The idea is that get_balance() is
slow, so we want to perform it under only a read lock, to avoid stalling
readers unnecessarily. However your code performs it again under the write
lock anyway. It's much simpler to just do the whole operation under a write
lock.

These examples are mostly what led me to believe that promotion isn't very
useful. But I may be missing an important use case.

Demotion, on the other hand, seems useful:

int deposit( int amount )
// requires: amount > 0
// returns: new balance (atomic)
{
    write_lock wr( m );
    v.push_back( amount );
    read_lock rd( wr );
    int r = 0;
    accumulate( v.begin(), v.end(), r );
    return r;
}