Subject: Re: INFO: suspicious rcu_dereference_check() usage.

From: Dave Jones <[email-masked]>
Date: Tue, 24 May 2011 11:56:09 -0400

On Mon, May 23, 2011 at 11:50:46PM -0700, Justin Mattock wrote:
 
 > [ 2862.310349] WARNING: at net/ipv4/route.c:1668 ip_rt_bug+0x5c/0x62()

Awesome, adding that WARN_ON paid off. This is the same bug I've been trying
to reproduce the last few weeks. DaveM mentioned that it means we used
an input route for packet output.

 > [ 2862.310414] Pid: 6153, comm: gcm-session Not tainted
 > 2.6.39-04906-g5e152b4-dirty #2
 > [ 2862.310417] Call Trace:
 > [ 2862.310424]  [<ffffffff8104c634>] warn_slowpath_common+0x83/0x9b
 > [ 2862.310430]  [<ffffffff8104c666>] warn_slowpath_null+0x1a/0x1c
 > [ 2862.310434]  [<ffffffff814095c9>] ip_rt_bug+0x5c/0x62
 > [ 2862.310439]  [<ffffffff814112a1>] dst_output+0x19/0x1d
 > [ 2862.310443]  [<ffffffff81412aa0>] ip_local_out+0x20/0x25
 > [ 2862.310448]  [<ffffffff814139c9>] ip_send_skb+0x19/0x58
 > [ 2862.310453]  [<ffffffff8142fa4e>] udp_send_skb+0x239/0x29b
 > [ 2862.310458]  [<ffffffff814310f0>] udp_sendmsg+0x5a1/0x7d4
 > [ 2862.310464]  [<ffffffff81079408>] ? trace_hardirqs_off+0xd/0xf
 > [ 2862.310469]  [<ffffffff8141139c>] ? ip_select_ident+0x3d/0x3d
 > [ 2862.310475]  [<ffffffff810525b8>] ? local_bh_enable_ip+0xe/0x10
 > [ 2862.310481]  [<ffffffff8148f131>] ? _raw_spin_unlock_bh+0x31/0x35
 > [ 2862.310486]  [<ffffffff813d41a6>] ? release_sock+0x14c/0x155
 > [ 2862.310490]  [<ffffffff814386ac>] inet_sendmsg+0x66/0x6f
 > [ 2862.310495]  [<ffffffff813d02b0>] sock_sendmsg+0xe6/0x109
 > [ 2862.310501]  [<ffffffff8107d63f>] ? lock_acquire+0xe1/0x109
 > [ 2862.310505]  [<ffffffff8107d535>] ? lock_release+0x1aa/0x1d3
 > [ 2862.310512]  [<ffffffff810ed549>] ? might_fault+0xa5/0xac
 > [ 2862.310516]  [<ffffffff813ceb34>] ? copy_from_user+0x2f/0x31
 > [ 2862.310521]  [<ffffffff813d1f34>] sys_sendto+0x132/0x174
 > [ 2862.310526]  [<ffffffff81495cfa>] ? sysret_check+0x2e/0x69
 > [ 2862.310531]  [<ffffffff8107b016>] ? trace_hardirqs_on_caller+0x13f/0x172
 > [ 2862.310537]  [<ffffffff8109fd4d>] ? audit_syscall_entry+0x11c/0x148
 > [ 2862.310542]  [<ffffffff8121debe>] ? trace_hardirqs_on_thunk+0x3a/0x3f
 > [ 2862.310546]  [<ffffffff81495cc2>] system_call_fastpath+0x16/0x1b
 > [ 2862.310549] ---[ end trace 2d2332adaa8bf2b5 ]---
 > [ 2863.373889] ip_rt_bug: 10.0.0.10 -> 255.255.255.255, ?
 
The common thing between your bug and the trace I triggered was the
destination ip reported. A clue maybe ?

	Dave