linux-kernel - Re: [PATCH 03/10] mac80211: Use rcu

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Message-Id: <1245838862.21314.48.camel@johannes.local>
Date:	Wed, 24 Jun 2009 12:21:02 +0200
From:	Johannes Berg <johannes@...solutions.net>
To:	Jesper Dangaard Brouer <hawk@...x.dk>
Cc:	"David S. Miller" <davem@...emloft.net>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
	dougthompson@...ssion.com, bluesmoke-devel@...ts.sourceforge.net,
	axboe@...nel.dk, Patrick McHardy <kaber@...sh.net>,
	christine.caulfield@...glemail.com, Trond.Myklebust@...app.com,
	linux-wireless@...r.kernel.org, yoshfuji@...ux-ipv6.org,
	shemminger@...ux-foundation.org, linux-nfs@...r.kernel.org,
	bfields@...ldses.org, neilb@...e.de, linux-ext4@...r.kernel.org,
	tytso@....edu, adilger@....com, netfilter-devel@...r.kernel.org
Subject: Re: [PATCH 03/10] mac80211: Use rcu_barrier() on unload.

On Wed, 2009-06-24 at 12:06 +0200, Jesper Dangaard Brouer wrote:
> On Tue, 2009-06-23 at 17:15 +0200, Johannes Berg wrote:
> > On Tue, 2009-06-23 at 17:04 +0200, Jesper Dangaard Brouer wrote:
> > > The mac80211 module uses rcu_call() thus it should use rcu_barrier()
> > > on module unload.
> > > 
> > > I'm having a hardtime verifying that no more call_rcu() callbacks can
> > > be schedules in the module unload path.  Could a maintainer please
> > > look into this?
> > > 
> > > Signed-off-by: Jesper Dangaard Brouer <hawk@...x.dk>
> > > ---
> > > 
> > >  net/mac80211/main.c |    2 ++
> > >  1 files changed, 2 insertions(+), 0 deletions(-)
> > > 
> > > diff --git a/net/mac80211/main.c b/net/mac80211/main.c
> > > index 092a017..e9f70ce 100644
> > > --- a/net/mac80211/main.c
> > > +++ b/net/mac80211/main.c
> > > @@ -1100,6 +1100,8 @@ static void __exit ieee80211_exit(void)
> > >  		ieee80211s_stop();
> > >  
> > >  	ieee80211_debugfs_netdev_exit();
> > > +
> > > +	rcu_barrier(); /* Wait for completion of call_rcu()'s */
> > >  }
> > 
> > I don't think this is correct at all -- note that call_rcu() is done in
> > some of the mesh code, so I would think you need to do this in
> > ieee80211_stop() since the call_rcu() code requires the interface to
> > still be around. And when it's stopped everything should be idle.
> 
> Should it then not be in mesh.c ieee80211_stop_mesh().  We can replace
> the synchronize_rcu() in this function with a rcu_barrier().

Yes, this seems correct.

johannes

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