[cpp-threads] Weak vs Strong Atomics

[Boehm, Hans]

> -----Original Message-----
> From: cpp-threads-bounces at decadentplace.org.uk 
> [mailto:cpp-threads-bounces at decadentplace.org.uk] On Behalf 
> Of Roger Orr
> Sent: Thursday, May 03, 2007 12:01 PM
> To: 'C++ threads standardisation'; c++std-ext at accu.org
> Subject: RE: [cpp-threads] Weak vs Strong Atomics
> 
>  
> > First, I don't think the notion that you can pass a weak 
> atomic to a 
> > module expecting a strong atomic actually works.
> > The passee may be expecting SC semantics, but the passer 
> may still be 
> > using it weakly, thus failing SC.  Conversely, as noted, passing a 
> > strong atomic to a weak atomic also may yield non-SC 
> behavior to the 
> > passer.  So, at a minimum, meeting the stated goal of the decision 
> > requires complete isolation of the weak atomics from the strong 
> > atomics, and that was not my understanding of the decision.
> 
> That matches my understanding that we were proposing a class 
> hierarchy, where weak_atomic was derived from strong_atomic, 
> as we thought a weak atomic IS-A strong atomic.
> 
> I think your first sentence shows that assumption is invalid.
> 
I can't quite convince myself of this.

Assume I have a weak atomic x in A, and I pass it to B as a strong
atomic.  A continues to apply weak operations to it while B uses it.
Does that allow any behavior that wouldn't already be exhibited by the
case in which A runs a few threads repeatedly executing the strong
operation

x = random();

in a tight loop?

It seems to me that whether or not A gets to apply weak operations, it
has to behave such that B sees correct values for x.  There are no doubt
more ways to get that wrong with weak operations.  But I have a hard
time pinning down any sense in which x fundamentally does not look like
a strong atomic to B.

Having said that, I don't have a strong opinion on this issue.  I think
either approach works for the usage models I expect to see:

- Only strong atomics
- Only weak atomics
- Coding initially with strong atomics and making isolated use of weak
atomics for tuning

But I would prefer not to reopen this discussion if it does turn out to
be controversial.

Hans