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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.


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