Subject: Re: [PATCH 03/10] mac80211: Use rcu_barrier() on unload.
From: Johannes Berg <johannes@sipsolutions.net>
To: Jesper Dangaard Brouer <hawk@comx.dk>
Cc: "David S. Miller" <davem@davemloft.net>,
       "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>, netdev@vger.kernel.org,
       linux-kernel@vger.kernel.org, dougthompson@xmission.com,
       bluesmoke-devel@lists.sourceforge.net, axboe@kernel.dk,
       Patrick McHardy <kaber@trash.net>, christine.caulfield@googlemail.com,
       Trond.Myklebust@netapp.com, linux-wireless@vger.kernel.org,
       yoshfuji@linux-ipv6.org, shemminger@linux-foundation.org,
       linux-nfs@vger.kernel.org, bfields@fieldses.org, neilb@suse.de,
       linux-ext4@vger.kernel.org, tytso@mit.edu, adilger@sun.com,
       netfilter-devel@vger.kernel.org
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Date: Tue, 23 Jun 2009 17:15:55 +0200
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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.
>=20
> 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?
>=20
> Signed-off-by: Jesper Dangaard Brouer <hawk@comx.dk>
> ---
>=20
>  net/mac80211/main.c |    2 ++
>  1 files changed, 2 insertions(+), 0 deletions(-)
>=20
> 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();
> =20
>  	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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