next-20131218 - WARNING in qdisc_list_add

[Valdis Kletnieks]

Seen once while booting next-20131218 (different boot than the BUG
I hit). Command that triggered it would have been one of these 2:

/usr/sbin/tc qdisc add dev em1 root codel
/usr/sbin/tc qdisc add dev wlp3s0 root codel

It's possible that wlp3s0 wasn't there yet due to rfkill switch having
nuked the wireless.

'git blame' points at the message being added in this commit:

commit e57a784d8cae429f5b697fe55abf420181d9ff09
Author: Eric Dumazet <edumazet@xxxxxxxxxx>
Date:   Thu Dec 12 15:41:56 2013 -0800

    pkt_sched: set root qdisc before change() in attach_default_qdiscs()

    After commit 95dc19299f74 ("pkt_sched: give visibility to mq slave
    qdiscs") we call disc_list_add() while the device qdisc might be
    the noop_qdisc one.

    This shows up as duplicates in "tc qdisc show", as all inactive devices
    point to noop_qdisc.

    Fix this by setting dev->qdisc to the new qdisc before calling
    ops->change() in attach_default_qdiscs()

    Add a WARN_ON_ONCE() to catch any future similar problem.

    Signed-off-by: Eric Dumazet <edumazet@xxxxxxxxxx>
    Signed-off-by: David S. Miller <davem@xxxxxxxxxxxxx>


 [  153.876539] ------------[ cut here ]------------
 [  153.878667] WARNING: CPU: 1 PID: 1446 at net/sched/sch_api.c:278 qdisc_list_add+0x79/0xcf()
 [  153.897042] CPU: 3 PID: 1446 Comm: tc Tainted: G           O 3.13.0-rc4-next-20131218 #129
 [  153.898877] Hardware name: Dell Inc. Latitude E6530/07Y85M, BIOS A11 03/12/2013
 [  153.900666]  0000000000000000 ffff880121195950 ffffffff8164c74f 0000000000000000
 [  153.905666]  ffff880121195988 ffffffff8103875e ffffffff814afe73 ffff88012828f600
 [  153.910847]  0000000000000001 ffffffff81c8cac0 0000000000000001 ffff880121195998
 [  153.916012] Call Trace:
 [  153.917237]  [<ffffffff8164c74f>] dump_stack+0x4f/0xa2
 [  153.918659]  [<ffffffff8103875e>] warn_slowpath_common+0x7a/0x93
 [  153.920222]  [<ffffffff814afe73>] ? qdisc_list_add+0x79/0xcf
 [  153.921896]  [<ffffffff81038816>] warn_slowpath_null+0x15/0x17
 [  153.923579]  [<ffffffff814afe73>] qdisc_list_add+0x79/0xcf
 [  153.925134]  [<ffffffff814b1466>] qdisc_create+0x237/0x350
 [  153.926658]  [<ffffffff814b1a05>] tc_modify_qdisc+0x486/0x4af
 [  153.928173]  [<ffffffff814a2bb0>] rtnetlink_rcv_msg+0x180/0x193
 [  153.929753]  [<ffffffff810c7c32>] ? trace_preempt_on+0x12/0x2f
 [  153.931453]  [<ffffffff814a2a30>] ? rtnl_newlink+0x4ba/0x4ba
 [  153.933107]  [<ffffffff814b67e4>] netlink_rcv_skb+0x44/0x86
 [  153.934586]  [<ffffffff8149fbeb>] rtnetlink_rcv+0x1e/0x25
 [  153.936133]  [<ffffffff814b62ef>] netlink_unicast+0xf1/0x175
 [  153.937587]  [<ffffffff814b6677>] netlink_sendmsg+0x304/0x34e
 [  153.939088]  [<ffffffff814794ac>] __sock_sendmsg_nosec+0x25/0x27
 [  153.940638]  [<ffffffff8147b0c2>] sock_sendmsg+0x52/0x6c
 [  153.942278]  [<ffffffff811009ce>] ? might_fault+0x9b/0x9f
 [  153.943980]  [<ffffffff81100985>] ? might_fault+0x52/0x9f
 [  153.945439]  [<ffffffff81487cec>] ? verify_iovec+0x5e/0xac
 [  153.946993]  [<ffffffff8147cd41>] ___sys_sendmsg+0x20d/0x29d
 [  153.948540]  [<ffffffff81073236>] ? arch_local_irq_save+0x9/0xc
 [  153.950087]  [<ffffffff81072708>] ? up_read+0x22/0x25
 [  153.951743]  [<ffffffff8165a21b>] ? __do_page_fault+0x56f/0x600
 [  153.953495]  [<ffffffff810c7bdd>] ? time_hardirqs_on+0x1b/0x2f
 [  153.955175]  [<ffffffff8113fdee>] ? fcheck_files+0x80/0xe0
 [  153.956720]  [<ffffffff8114062d>] ? fget_light+0x30/0x90
 [  153.958195]  [<ffffffff8147d20a>] __sys_sendmsg+0x3d/0x5b
 [  153.959645]  [<ffffffff8147d235>] SyS_sendmsg+0xd/0x17
 [  153.961092]  [<ffffffff8165d722>] system_call_fastpath+0x16/0x1b
 [  153.965229] ---[ end trace ec29ec2eecd5a461 ]---

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