Subj : Re: Can C++ local static objects be made thread safe?
To   : comp.programming.threads
From : David Schwartz
Date : Sat Jan 22 2005 05:53 pm


"Gianni Mariani" <gi2nospam@mariani.ws> wrote in message 
news:4sadnW1IWuV3UGzcRVn-vg@speakeasy.net...

> On the first time through, a memory barrier sync is done.  Worst case, you 
> can't come out of this code normally without the construction being 
> completed. Once you decide that you're locking the mutex, I assume 
> barriers are looked after.  The m_val value is thread local, meaning each 
> thread executed the memory barrier at least once - once the memory barrier 
> is synced once, why would it need to be synced again ?  If it did, 
> somthing much more serious is broken.

    Ah, I missed that m_val is thread local. The problem with this routine 
is that each pass through it requires access to a thread local variable. 
This may even cost more than a memory barrier on many platforms, and it's 
likely more efficient to just have the memory barrier be unconditional.

    DS

.