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Date:	Tue, 27 Jul 2010 05:38:59 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Johannes Berg <johannes@...solutions.net>
Cc:	netdev <netdev@...r.kernel.org>
Subject: Re: br_forward.c - rcu dereference warning

On Tue, Jul 27, 2010 at 12:45:53PM +0200, Johannes Berg wrote:
> I couldn't find this reported yet, apologies if I missed it.

My first guess is that the call to br_multicast_deliver() in
br_dev_xmit() needs to be enclosed in rcu_read_lock(), but I have
to defer to someone who knows the code better.  Another possible
fix would be to change the rcu_dereference() in br_multicast_flood()
to rcu_dereference_bh(), for example.

						Thanx, Paul

> johannes
> 
> [   60.140433] ===================================================
> [   60.140437] [ INFO: suspicious rcu_dereference_check() usage. ]
> [   60.140440] ---------------------------------------------------
> [   60.140444] /home/johannes/sys/wireless-testing/net/bridge/br_forward.c:215 invoked rcu_dereference_check() without protection!
> [   60.140447] 
> [   60.140448] other info that might help us debug this:
> [   60.140449] 
> [   60.140452] 
> [   60.140453] rcu_scheduler_active = 1, debug_locks = 1
> [   60.140457] 2 locks held by Xorg/3083:
> [   60.140459]  #0:  (&im->timer){+.-...}, at: [<ffffffff81058180>] call_timer_fn+0x0/0x2f0
> [   60.140473]  #1:  (rcu_read_lock_bh){.+....}, at: [<ffffffff813b6c3a>] dev_queue_xmit+0x5a/0x690
> [   60.140484] 
> [   60.140484] stack backtrace:
> [   60.140489] Pid: 3083, comm: Xorg Not tainted 2.6.35-rc6-wl-47665-gc2e2180-dirty #174
> [   60.140492] Call Trace:
> [   60.140495]  <IRQ>  [<ffffffff8107e494>] lockdep_rcu_dereference+0xa4/0xc0
> [   60.140514]  [<ffffffffa07617c3>] br_multicast_flood+0x293/0x310 [bridge]
> [   60.140531]  [<ffffffffa0761877>] br_multicast_deliver+0x17/0x20 [bridge]
> [   60.140539]  [<ffffffffa07607bc>] br_dev_xmit+0x10c/0x170 [bridge]
> [   60.140550]  [<ffffffff813b69ba>] dev_hard_start_xmit+0x21a/0x2e0
> [   60.140556]  [<ffffffff813b708e>] dev_queue_xmit+0x4ae/0x690
> [   60.140576]  [<ffffffff813c0d43>] neigh_resolve_output+0x113/0x250
> [   60.140582]  [<ffffffff813eec46>] ip_finish_output+0x2a6/0x570
> [   60.140588]  [<ffffffff813ef33c>] ip_mc_output+0x1dc/0x320
> [   60.140593]  [<ffffffff813ed7ed>] ip_local_out+0x2d/0x80
> [   60.140600]  [<ffffffff81422236>] igmp_send_report+0x1c6/0x200
> [   60.140610]  [<ffffffff814238f0>] igmp_timer_expire+0x100/0x130
> [   60.140615]  [<ffffffff81058219>] call_timer_fn+0x99/0x2f0
> [   60.140636]  [<ffffffff810585e3>] run_timer_softirq+0x173/0x330
> [   60.140641]  [<ffffffff8104f114>] __do_softirq+0x114/0x3d0
> [   60.140652]  [<ffffffff8100360c>] call_softirq+0x1c/0x50
> 
> 
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