Re: [Boost-users] [Thread] How to invoke a memory fence?

Subject: Re: [Boost-users] [Thread] How to invoke a memory fence?
From: Radosavljevic, Branko (branko.radosavljevic_at_[hidden])
Date: 2009-01-29 13:07:57

> "Radosavljevic, Branko" <branko.radosavljevic_at_[hidden]>
> writes:
> 
> > I have a newbie question about using Boost.Thread to invoke a memory
> > fence operation.  What are the memory fence guarantees associated
> > with the synchronization concepts in Boost.Thread?
> >
> > My specific use case involves one writer and several readers.
> > For example, something like:
> >
> > bool available = false;
> > int value;
> >
> > // Writer thread
> > value = getValue();
> > available = true;
> >
> > // A reader thread
> > while (!available)
> >     yield();
> > doSomething(value);
> >
> > In other words, the write to available should have release
> > semantics, and the read from available should have acquire
> > semantics.  I realize I can use platform-specific commands,
> > but I'd rather not.  I assume that if I want to use Boost.Thread,
> > I'll need to use a mutex, but I don't see any discussion
> > of memory fence issues in the documentation.
> 

Anthony Williams [anthony.ajw_at_[hidden]] writes:
> If you use a mutex to protect "available", then the appropriate
> semantics will be in place.

Zeljko Vrba [zvrba_at_[hidden]] writes:
> POSIX mutexes gurantee that proper fences will be applied at 
> the right spots.
> My guess is that the same holds also on Windows, but you 
> should check with
> the documentation. (If you're using x86/x64, all atomic 
> instructions are also
> acquire/release fences.)

Thanks, Anthony and Zeljko.  It appears that any mutex lock and
unlock will apply the correct memory fences, so that for this
purpose only, the writer and readers can use separate mutexes
(ie, guaranteed no contention).  At least, I verified this
for Windows.  However, the separate issue of ensuring that
the read of "available" is not corrupt, in case there's a
simultaneous write, would require that the reader and writer
use the same mutex.  That's the approach that I'll use.

Thanks again,
Branko

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