From owner-freebsd-smp  Tue Oct 31 10:21:14 2000
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Date: Tue, 31 Oct 2000 10:21:10 -0800
From: Alfred Perlstein <bright@wintelcom.net>
To: Cedric Berger <cedric@wireless-networks.com>
Cc: Robert Watson <rwatson@FreeBSD.ORG>, freebsd-smp@FreeBSD.ORG
Subject: Re: Reference count invariants in a fine-grained threaded environment
Message-ID: <20001031102110.V22110@fw.wintelcom.net>
References: <Pine.NEB.3.96L.1001031123647.58688x-100000@fledge.watson.org> <39FF0AC0.86A3A950@wireless-networks.com>
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* Cedric Berger <cedric@wireless-networks.com> [001031 10:11] wrote:
> 
> 
> Robert Watson wrote:
> > 
> [...]
> > 
> > Rules for interactions between mutexes and reference-counted kernel
> > objects:
> > 
> > Assumptions:
> > 
> > - Objects with reference counts have a mutex that can protect their
> >   reference count, and possibly other variables (or other instances).  For
> >   example, struct cred might have a mutex per instance, but all struct
> >   prison's might use the same mutex.
> > 
> 
> Is there not a cheaper way to manage reference count then using mutexes?
> I would guess that all architectures must have hardware support (i.e. special
> assembly instruction) to atomically increment/decrement+read a 32-bit value
> in a multiprocessor environment

Yes, but FreeBSD is has too many knights that say NIH!

This is where atomic refcounts would simplify the code however for some
reason everyone I've talked to prefers to have this sort of code:



void
crhold(struct ucred* ucp) {

	mtx_lock(ucp->uc_mtx);
	ucp->ref++;
	mtx_unlock(ucp->uc_mtx);
}

rather than a simple and cheaper:

void
crhold(struct ucred* ucp) {
  
        atomic_ref_inc(&ucp->ref);
} 

*shrug*

-- 
-Alfred Perlstein - [bright@wintelcom.net|alfred@freebsd.org]
"I have the heart of a child; I keep it in a jar on my desk."


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