Subj : sem_getvalue  (was Re: Using hierarchical memory ...)
To   : comp.programming.threads,comp.arch
From : Joe Seigh
Date : Mon Sep 12 2005 03:00 pm

(followup set to comp.programming.threads)

Alexander Terekhov wrote:
> Joe Seigh wrote:
> [...]
> 
>>I try not to conflate specification and implementation.
> 
> 
> Conflation or non-conflation aside for a moment, I'm just curious how 
> your lock-free sem_getvalue() for x86 would look like. Care to share?
> 

Depends.  The documentation for that is a little strange and it's not
clear if sem_getvalue has any practical use.

To be on the safe side you might have to use the same synchronization
that the rest of the semaphore implementation uses to set the semaphore
value.  Though I might be inclined, if it looks like all you could use
sem_getvalue for is as a simple event synchronization object, to just
to do a simple load followed by a mfence, acquire semantics.  It's basically
a question of what the observable behavior is no matter what the
documentation says.

I see you've been looking for places to use your compare and
swap "synchronized" load trick.  :)



-- 
Joe Seigh

When you get lemons, you make lemonade.
When you get hardware, you make software.

.