linux-kernel - Re: [PATCH 2/2] auth_gss: unregister gss

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Date:	Mon, 30 Oct 2006 10:17:29 -0500
From:	Trond Myklebust <Trond.Myklebust@...app.com>
To:	Akinobu Mita <akinobu.mita@...il.com>
Cc:	linux-kernel@...r.kernel.org, Andy Adamson <andros@...i.umich.edu>,
	"J. Bruce Fields" <bfields@...i.umich.edu>,
	Olaf Kirch <okir@...ad.swb.de>
Subject: Re: [PATCH 2/2] auth_gss: unregister gss_domain when unloading
	module

On Mon, 2006-10-30 at 23:54 +0900, Akinobu Mita wrote:
> On Sun, Oct 29, 2006 at 03:15:59PM -0500, Trond Myklebust wrote:
> 
> > > +void svcauth_gss_unregister_pseudoflavor(char *name)
> > > +{
> > > +	struct auth_domain *dom;
> > > +
> > > +	dom = auth_domain_find(name);
> > > +	if (dom) {
> > > +		auth_domain_put(dom);
> > > +		auth_domain_put(dom);
> > > +	}
> > > +}
> > 
> > Strictly speaking, if you want to be smp-safe, you probably need
> > something like the following:
> > 
> > 	dom = auth_domain_find(name);
> > 	if (dom) {
> > 		spin_lock(&auth_domain_lock);
> > 		if (!hlist_unhashed(dom->hash)) {
> > 			hlist_del_init(dom->hash);
> > 			spin_unlock(&auth_domain_lock);
> > 			auth_domain_put(dom);
> > 		} else
> > 			spin_unlock(&auth_domain_lock);
> > 		auth_domain_put(dom);
> > 	}
> > 
> > and then add a test for hlist_unhashed into auth_domain_put(). If not,
> > some other processor could race you inside
> > svcauth_gss_unregister_pseudoflavor.
> 
> But auth_domain_table is protected by auth_domain_lock while we are
> using auth_domain_put()/auth_domain_lookup()/auth_domain_find().
> So I think there is not big difference.

No. The auth_domain_lock was released after the call to
auth_domain_find(), and thus there is no guarantee that the entry is
still referenced when you get round to that second call to
auth_domain_put(). Testing for hlist_unhashed() and then removing the
entry from the lookup table while under the spin lock fixes this
problem: it ensures that you only call auth_domain_put() once if some
other process has raced you.

Actually, making a helper "auth_domain_find_and_unhash()" would probably
be even more efficient in this case, since that would enable you to
reduce the auth_domain_lock usage further.

Cheers,
  Trond
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