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

From: Howard Hinnant (hinnant_at_[hidden])
Date: 2004-07-10 14:51:43

On Jul 9, 2004, at 9:47 PM, Matt Hurd wrote:

> From memory the use case was reading stuff and then if that said I
> needed to write something, write it by upgrading the lock.
>
> However, you are not going to be able to escape the deadlock in this
> case _ever_.  AFAICT was if two things read concurrently and need to
> change state, the both can't if the states are dependent and thus you
> always have a deadlock.

The upgradable_read_lock, not abused as in the case I showed, will not 
deadlock.  The key (and the restriction) is that only one 
upgradable_read_lock can be active at a time, though it can share with 
any number of read_lock's.  You are right about the 2-read-to-write 
promotion case.  And that is exactly why upgradable_read_lock's are 
exclusive among themselves.  And it is also why you can't "trick" a 
read_lock into a promotion (as my abuse-example showed).

I believe the upgradable_read_lock is still a viable solution for a 
very special situation, but it requires suitable documentation to guide 
the programmer in how not to abuse it.

-Howard