linux-kernel - Re: [PATCH 3/5] can: af_can.c use rcu

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Date:	Mon, 8 Jun 2009 09:13:29 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Jesper Dangaard Brouer <hawk@...x.dk>
Cc:	"David S. Miller" <davem@...emloft.net>, netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org, urs.thuermann@...kswagen.de,
	oliver.hartkopp@...kswagen.de, wg@...ndegger.com,
	vladislav.yasevich@...com, sri@...ibm.com,
	linux-sctp@...r.kernel.org, Trond.Myklebust@...app.com,
	linux-nfs@...r.kernel.org, netfilter-devel@...r.kernel.org
Subject: Re: [PATCH 3/5] can: af_can.c use rcu_barrier() on module unload.

On Mon, Jun 08, 2009 at 03:11:38PM +0200, Jesper Dangaard Brouer wrote:
> This module uses rcu_call() thus it should use rcu_barrier()
> on module unload.

This does appear to make things better!!!

However, I don't understand why it is safe to do the following in
can_exit():

	hlist_for_each_entry_safe(d, n, next, &can_rx_dev_list, list) {
		hlist_del(&d->list);
		kfree(d);
	}

Given that this list is scanned by RCU readers, shouldn't this kfree()
be something like "call_rcu(&d->rcu, can_rx_delete_device);"?

Also, what frees up the "struct receiver" structures?

							Thanx, Paul

> Signed-off-by: Jesper Dangaard Brouer <hawk@...x.dk>
> ---
> 
>  net/can/af_can.c |    2 ++
>  1 files changed, 2 insertions(+), 0 deletions(-)
> 
> diff --git a/net/can/af_can.c b/net/can/af_can.c
> index 10f0528..e733725 100644
> --- a/net/can/af_can.c
> +++ b/net/can/af_can.c
> @@ -903,6 +903,8 @@ static __exit void can_exit(void)
>  	}
>  	spin_unlock(&can_rcvlists_lock);
> 
> +	rcu_barrier(); /* Wait for completion of call_rcu()'s */
> +
>  	kmem_cache_destroy(rcv_cache);
>  }
> 
> 
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