linux-kernel - Re: [PATCH 1/3] net/bridge/br_if.c: don't use _WORK

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Message-ID: <20070219115955.GB91@tv-sign.ru>
Date:	Mon, 19 Feb 2007 14:59:55 +0300
From:	Oleg Nesterov <oleg@...sign.ru>
To:	David Howells <dhowells@...hat.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Jarek Poplawski <jarkao2@...pl>,
	"David S. Miller" <davem@...emloft.net>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/3] net/bridge/br_if.c: don't use _WORK_NAR

On 02/19, David Howells wrote:
>
> Oleg Nesterov <oleg@...sign.ru> wrote:
> 
> > Afaics, noautorel work_struct buys nothing for "struct net_bridge_port".
> 
> You may be right.
> 
> > If del_nbp()->cancel_delayed_work(&p->carrier_check) fails, port_carrier_check
> > may be called later anyway.
> 
> Called by what?  Something outside of br_if.c?

No. if cancel_delayed_work() fails, the work may sit pending in cwq->worklist,
or it may be running right now, waiting for rtnl_mutex.

> > So the reading of *work in port_carrier_check() is equally unsafe with or
> > without this patch.
> 
> Hmmm... cancel_delayed_work() in del_nbp() probably ought to be followed by a
> flush_scheduled_work().

Yes, but this deadlocks: we hold rtnl_mutex, and work->func() takes it too.

I think the fix should be so that port_carrier_check() does get/put on
"struct net_bridge_port" (container), but not on "struct net_device", and

del_nbp(struct net_bridge_port *p)

	if (cancel_delayed_work(&p->carrier_check))
-		dev_put(p->dev);
+		kobject_put(&p->kobj);

Oleg.

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