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

From: Peter Dimov (pdimov_at_[hidden])
Date: 2004-06-29 12:00:47

Michael Glassford wrote:
>
> Also, you could end up with some interesting situations like this:
>
>      void f(read_write_mutex m)
>      {
>          read_write_mutex::read_lock r(m);
>          if (...)
>          {
>              read_write_mutex::write_lock w(r); //lock promotion
>              //...
>          }
>          //Point A
>      }
>
> The most obvious implementation of promotion would be for the write lock
> to unlock the read lock if promotion succeeded, but leave it locked if
> promotion failed. But in the above code, this would mean that if
> promotion succeeds, neither lock will be locked at "Point A"; however
> if promotion fails, r will still be read-locked at point A.

Not necessarily, ~write_lock can (should) demote the lock back to read (or
whatever the initial condition of r was).