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).