Subject: Re: [PATCH] RCU: don't turn off lockdep when find suspicious rcu_dereference_check() usage

From: "Paul E. McKenney" <[email-masked]>
Date: Wed, 21 Apr 2010 14:45:42 -0700

On Wed, Apr 21, 2010 at 08:04:28AM +0200, Borislav Petkov wrote:
> Hi,
> 
> a plain -rc5 triggers at net/core/dev.c:1993 here too:

Thank you for testing this, Borislav!  A prototype patch may be
found below, CCing the netdev guys for their thoughts on this.
(And if this patch is valid, it is probably best for it to go up with
the networking patches, since this part of the code looks to be under
active development.)

							Thanx, Paul

> [   12.889090] ===================================================
> [   12.889387] [ INFO: suspicious rcu_dereference_check() usage. ]
> [   12.889533] ---------------------------------------------------
> [   12.889679] net/core/dev.c:1993 invoked rcu_dereference_check() without protection!
> [   12.889929] 
> [   12.889929] other info that might help us debug this:
> [   12.889930] 
> [   12.890368] 
> [   12.890369] rcu_scheduler_active = 1, debug_locks = 0
> [   12.890659] 2 locks held by swapper/0:
> [   12.890803]  #0:  (&idev->mc_ifc_timer){+.-...}, at: [<ffffffff81045f4a>] run_timer_softirq+0x266/0x503
> [   12.891227]  #1:  (rcu_read_lock_bh){.+....}, at: [<ffffffff81397eb4>] dev_queue_xmit+0x153/0x512
> [   12.891647] 
> [   12.891648] stack backtrace:
> [   12.891934] Pid: 0, comm: swapper Not tainted 2.6.34-rc5 #1
> [   12.892085] Call Trace:
> [   12.892231]  <IRQ>  [<ffffffff81065d8f>] lockdep_rcu_dereference+0xaa/0xb2
> [   12.892430]  [<ffffffff81397fbf>] dev_queue_xmit+0x25e/0x512
> [   12.892576]  [<ffffffff81397eb4>] ? dev_queue_xmit+0x153/0x512
> [   12.892723]  [<ffffffff81066a4a>] ? trace_hardirqs_on+0xd/0xf
> [   12.892871]  [<ffffffff8103f4fb>] ? local_bh_enable_ip+0xbc/0xda
> [   12.893024]  [<ffffffff8139ea67>] neigh_resolve_output+0x323/0x36a
> [   12.893183]  [<ffffffffa00ae6b7>] ? ipv6_chk_mcast_addr+0x0/0x1fa [ipv6]
> [   12.893338]  [<ffffffffa0094860>] ip6_output_finish+0x81/0xb9 [ipv6]
> [   12.893492]  [<ffffffffa0096067>] ip6_output2+0x2a9/0x2b4 [ipv6]
> [   12.893644]  [<ffffffffa0096c33>] ip6_output+0xbc1/0xbd0 [ipv6]
> [   12.893797]  [<ffffffffa00a2a06>] ? fib6_force_start_gc+0x30/0x32 [ipv6]
> [   12.893951]  [<ffffffffa00b04e8>] mld_sendpack+0x30b/0x435 [ipv6]
> [   12.894109]  [<ffffffffa00b01dd>] ? mld_sendpack+0x0/0x435 [ipv6]
> [   12.894264]  [<ffffffff8106676d>] ? mark_held_locks+0x52/0x70
> [   12.894418]  [<ffffffffa00b0d2d>] mld_ifc_timer_expire+0x254/0x28d [ipv6]
> [   12.894570]  [<ffffffff81046065>] run_timer_softirq+0x381/0x503
> [   12.894717]  [<ffffffff81045f4a>] ? run_timer_softirq+0x266/0x503
> [   12.894870]  [<ffffffffa00b0ad9>] ? mld_ifc_timer_expire+0x0/0x28d [ipv6]
> [   12.895024]  [<ffffffff8103f708>] ? __do_softirq+0x79/0x2f5
> [   12.895174]  [<ffffffff8103f80f>] __do_softirq+0x180/0x2f5
> [   12.895323]  [<ffffffff810030cc>] call_softirq+0x1c/0x28
> [   12.895472]  [<ffffffff81004d91>] do_softirq+0x3d/0x85
> [   12.895619]  [<ffffffff8103f2b5>] irq_exit+0x4a/0x95
> [   12.895766]  [<ffffffff81413a3d>] smp_apic_timer_interrupt+0x8c/0x9a
> [   12.895913]  [<ffffffff81002b93>] apic_timer_interrupt+0x13/0x20
> [   12.896065]  <EOI>  [<ffffffff81412e1e>] ? _raw_spin_unlock_irqrestore+0x38/0x69
> [   12.896363]  [<ffffffff8100a189>] ? default_idle+0xd8/0x10a
> [   12.896512]  [<ffffffff8100a187>] ? default_idle+0xd6/0x10a
> [   12.896658]  [<ffffffff8100a5b3>] c1e_idle+0xcd/0xf4
> [   12.896805]  [<ffffffff8100138c>] cpu_idle+0x5e/0xb5
> [   12.896952]  [<ffffffff813ff0eb>] rest_init+0xff/0x106
> [   12.897104]  [<ffffffff813fefec>] ? rest_init+0x0/0x106
> [   12.897260]  [<ffffffff818e7c2a>] start_kernel+0x30f/0x31a
> [   12.897409]  [<ffffffff818e726d>] x86_64_start_reservations+0x7d/0x81
> [   12.897560]  [<ffffffff818e7355>] x86_64_start_kernel+0xe4/0xeb

commit df3f17af2d26d1451a3d23d5c7b7a6423a38569e
Author: Paul E. McKenney <[email-masked]>
Date:   Wed Apr 21 14:40:37 2010 -0700

    net: fix dev_pick_tx() to use rcu_dereference_bh()
    
    dev_pick_tx() is called in an RCU-bh read-side critical section, but
    uses rcu_dereference().  This patch changes to rcu_dereference_bh() in
    order to suppress the RCU lockdep splat.
    
    Signed-off-by: Paul E. McKenney <[email-masked]>

diff --git a/net/core/dev.c b/net/core/dev.c
index 92584bf..f769098 100644
--- a/net/core/dev.c
+++ b/net/core/dev.c
@@ -1990,7 +1990,7 @@ static struct netdev_queue *dev_pick_tx(struct net_device *dev,
 				queue_index = skb_tx_hash(dev, skb);
 
 			if (sk) {
-				struct dst_entry *dst = rcu_dereference(sk->sk_dst_cache);
+				struct dst_entry *dst = rcu_dereference_bh(sk->sk_dst_cache);
 
 				if (dst && skb_dst(skb) == dst)
 					sk_tx_queue_set(sk, queue_index);