[boost] Re: Lock unification (shared vs. upgradable)

From: Alexander Terekhov (terekhov_at_[hidden])
Date: 2004-08-09 09:26:09

Howard Hinnant wrote:
[...]
> So I believe you could design code at least partly the way you've
> proposed with my current proposal.  Although I'm sure I'm not fully
> understanding the write_lock.upgrade_pending() functionality.  The way
> I currently have my rw_mutex implemented, this information is not
> known.  

I know that.

>         Actually my current implementation is based on your suggested
> "single entry gate" design.  So when the mutex is write locked, it has
> no idea who or if anyone is waiting outside the gate.  

You'd need a semaphore, not a mutex for "entry gate". (first 
upgrader/writer would lock it and the last one would unlock it).

>                                                        I liked the idea
> that the scheduler, not the mutex, decides if a reader or writer gets
> priority.  

Yes, but this scheme has a drawback. "Serial wakes across context 
switching" (in addition to locking across context switching). For 
alternative, see (for example)

http://groups.google.com/groups?selm=3D9196B2.9BC29299%40web.de
(Subject: Re: rwlock using pthread_cond)

regards,
alexander.