netdev - Re: [PATCH 3/4] 8139too: RTNL and flush_scheduled

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Date:	Mon, 19 Feb 2007 13:05:00 +0100
From:	Jarek Poplawski <jarkao2@...pl>
To:	Francois Romieu <romieu@...zoreil.com>
Cc:	jeff@...zik.org,
	Stephen Hemminger <shemminger@...ux-foundation.org>,
	akpm@...ux-foundation.org, netdev@...r.kernel.org,
	Ben Greear <greearb@...delatech.com>,
	Kyle Lucke <klucke@...ibm.com>,
	Raghavendra Koushik <raghavendra.koushik@...erion.com>,
	Al Viro <viro@....linux.org.uk>
Subject: Re: [PATCH 3/4] 8139too: RTNL and flush_scheduled_work deadlock

On Fri, Feb 16, 2007 at 09:20:34PM +0100, Francois Romieu wrote:
> Jarek Poplawski <jarkao2@...pl> :
...
> > > @@ -1603,18 +1605,21 @@ static void rtl8139_thread (struct work_struct *work)
> > >  	struct net_device *dev = tp->mii.dev;
> > >  	unsigned long thr_delay = next_tick;
> > >  
> > > +	rtnl_lock();
> > > +
> > > +	if (!netif_running(dev))
> > > +		goto out_unlock;
> > 
> > I wonder, why you don't do netif_running before
> > rtnl_lock ? It's an atomic operation. And I'm not sure if increasing
> > rtnl_lock range is really needed here.
> 
> thread    A: netif_running()
> user task B: rtnl_lock()
> user task B: dev->close()
> user task B: rtnl_unlock()
> thread    A: rtnl_lock()
> thread    A: mess with closed device
> 
> Btw, the thread runs every 3*HZ at most.

You are right (mostly)! But I think rtnl_lock is special
and should be spared (even this 3*HZ) and here it's used
for some mainly internal purpose (close synchronization).
And it looks like mainly for this internal reason holding
of rtnl_lock is increased. And because rtnl_lock is quite
popular you have to take into consideration that after
this 3*HZ it could spend some time waiting for the lock.
So, maybe it would be nicer to check this netif_running
twice (after rtnl_lock where needed), but maybe it's a
mater of taste only, and yours is better, as well.
(Btw. I didn't verify this, but I hope you checked that
places not under rtnl_lock before the patch are safe from
some locking problems now.)

Jarek P.
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