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

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Date:	Tue, 23 Jun 2009 17:15:55 +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 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.

I can fix it later, but I'm deep in some other stuff right now.

johannes

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