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	Fri, 2 May 1997 14:00:39 +0200 (CEST)
To: Peter Dufault <dufault@hda.com>
cc: phk@dk.tfs.com (Poul-Henning Kamp), current@freebsd.org
From: Poul-Henning Kamp <phk@dk.tfs.com>
Subject: Re: vnode->v_usage 
In-reply-to: Your message of "Fri, 02 May 1997 07:24:54 EDT."
             <199705021124.HAA27754@hda.hda.com> 
Date: Fri, 02 May 1997 14:00:39 +0200
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In message <199705021124.HAA27754@hda.hda.com>, Peter Dufault writes:

>The first unlock won't work since a second thread can come in and
>decide to do the same move.  Obviously you have a lock ordering issue
>with the nested lock and you have to define the ordering.

see below for my solution.

>Moving items between two lists should be atomic - it is common
>and lends itself to clean implementation.
It's actually the same list in this case...

>Aside: how about making the queues truly opaque while you're doing this?
>Then you have a single place to enable locking hierarchy violation detection,
>*CAS types of implementations, etc.

I don't think that would be very easy.  On the other hand, there
are very few pieces of code manipulating the vnode free list, do
it's hardly worth the effort.  Or did I misunderstand you ?

Poul-Henning

void
vtouch(vp)
	struct vnode *vp;
{
	simple_lock(&vp->v_interlock);
	if (vp->v_usecount) {
		simple_unlock(&vp->v_interlock);
		return;
	}
	if (simple_lock_try(&vnode_free_list_slock)) {
		TAILQ_REMOVE(&vnode_free_list, vp, v_freelist);
		TAILQ_INSERT_TAIL(&vnode_free_list, vp, v_freelist);
		simple_unlock(&vnode_free_list_slock);
	}
	simple_unlock(&vp->v_interlock);
}
--
Poul-Henning Kamp           | phk@FreeBSD.ORG       FreeBSD Core-team.
http://www.freebsd.org/~phk | phk@login.dknet.dk    Private mailbox.
whois: [PHK]                | phk@tfs.com           TRW Financial Systems, Inc.
Power and ignorance is a disgusting cocktail.