linux-kernel - Re: syzbot rcu/debugobjects warning

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Date:   Fri, 23 Mar 2018 21:41:28 +0100 (CET)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Joel Fernandes <joelaf@...gle.com>
cc:     Paul McKenney <paulmck@...ux.vnet.ibm.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Todd Poynor <toddpoynor@...gle.com>, netdev@...r.kernel.org
Subject: Re: syzbot rcu/debugobjects warning

On Fri, 23 Mar 2018, Joel Fernandes wrote:
> On Fri, Mar 23, 2018 at 2:11 AM, Thomas Gleixner <tglx@...utronix.de> wrote:
> > On Thu, 22 Mar 2018, Joel Fernandes wrote:
> Sorry. Here is the raw crash log: https://pastebin.com/raw/puvh0cXE
> (The kernel logs are toward the end with the above).

And that is interesting:

[  150.629667]  <IRQ> [  150.631700]  [<ffffffff81d96069>] dump_stack+0xc1/0x128
[  150.637051]  [<ffffffff81dfefb6>] ? __debug_object_init+0x526/0xc40
[  150.643431]  [<ffffffff8142fbd1>] panic+0x1bc/0x3a8
[  150.648416]  [<ffffffff8142fa15>] ? percpu_up_read_preempt_enable.constprop.53+0xd7/0xd7
[  150.656611]  [<ffffffff81430835>] ? load_image_and_restore+0xf9/0xf9
[  150.663070]  [<ffffffff81269efd>] ? vprintk_default+0x1d/0x30
[  150.668925]  [<ffffffff81131879>] ? __warn+0x1a9/0x1e0
[  150.674170]  [<ffffffff81dfefb6>] ? __debug_object_init+0x526/0xc40
[  150.680543]  [<ffffffff81131894>] __warn+0x1c4/0x1e0
[  150.685614]  [<ffffffff81131afc>] warn_slowpath_null+0x2c/0x40
[  150.691972]  [<ffffffff81dfefb6>] __debug_object_init+0x526/0xc40
[  150.698174]  [<ffffffff81dfea90>] ? debug_object_fixup+0x30/0x30
[  150.704283]  [<ffffffff81dff709>] debug_object_init_on_stack+0x19/0x20
[  150.710917]  [<ffffffff81287a93>] __wait_rcu_gp+0x93/0x1b0
[  150.716508]  [<ffffffff81290251>] synchronize_rcu.part.65+0x101/0x110
[  150.723054]  [<ffffffff81290150>] ? rcu_pm_notify+0xc0/0xc0
[  150.728735]  [<ffffffff81292bc0>] ? __call_rcu.constprop.72+0x910/0x910
[  150.735459]  [<ffffffff81235221>] ? __lock_is_held+0xa1/0xf0
[  150.741223]  [<ffffffff81290287>] synchronize_rcu+0x27/0x90

So this calls synchronize_rcu from a rcu callback. That's a nono. This is
on the back of an interrupt in softirq context and __wait_rcu_gp() can
sleep, which is obviously a bad idea in softirq context....

Cc'ed netdev ....

And that also explains the debug object splat because this is not running
on the task stack. It's running on the softirq stack ....

[  150.746908]  [<ffffffff83588b35>] __l2tp_session_unhash+0x3d5/0x550
[  150.753281]  [<ffffffff8358891f>] ? __l2tp_session_unhash+0x1bf/0x550
[  150.759828]  [<ffffffff8114596a>] ? __local_bh_enable_ip+0x6a/0xd0
[  150.766123]  [<ffffffff8358ddb0>] ? l2tp_udp_encap_recv+0xd90/0xd90
[  150.772497]  [<ffffffff83588e97>] l2tp_tunnel_closeall+0x1e7/0x3a0
[  150.778782]  [<ffffffff835897be>] l2tp_tunnel_destruct+0x30e/0x5a0
[  150.785067]  [<ffffffff8358965a>] ? l2tp_tunnel_destruct+0x1aa/0x5a0
[  150.791537]  [<ffffffff835894b0>] ? l2tp_tunnel_del_work+0x460/0x460
[  150.797997]  [<ffffffff82ee8053>] __sk_destruct+0x53/0x570
[  150.803588]  [<ffffffff81293918>] rcu_process_callbacks+0x898/0x1300
[  150.810048]  [<ffffffff812939f7>] ? rcu_process_callbacks+0x977/0x1300
[  150.816684]  [<ffffffff82ee8000>] ? __sk_dst_check+0x240/0x240
[  150.822625]  [<ffffffff838be5d6>] __do_softirq+0x206/0x951
[  150.828223]  [<ffffffff81147315>] irq_exit+0x165/0x190
[  150.833557]  [<ffffffff838bd1eb>] smp_apic_timer_interrupt+0x7b/0xa0
[  150.840018]  [<ffffffff838b9470>] apic_timer_interrupt+0xa0/0xb0
[  150.846132]  <EOI> [  150.848166]  [<ffffffff838b6756>] ? native_safe_halt+0x6/0x10
[  150.854036]  [<ffffffff8123bf2d>] ? trace_hardirqs_on+0xd/0x10
[  150.859973]  [<ffffffff838b5d85>] default_idle+0x55/0x360
[  150.865478]  [<ffffffff8106be0a>] arch_cpu_idle+0xa/0x10
[  150.870896]  [<ffffffff838b6b96>] default_idle_call+0x36/0x60
[  150.876751]  [<ffffffff81226cb0>] cpu_startup_entry+0x2b0/0x380
[  150.882787]  [<ffffffff81226a00>] ? cpu_in_idle+0x20/0x20
[  150.888291]  [<ffffffff812d2343>] ? clockevents_register_device+0x123/0x200
[  150.895358]  [<ffffffff810b0693>] start_secondary+0x303/0x3e0
[  150.901209]  [<ffffffff810b0390>] ? set_cpu_sibling_map+0x11f0/0x11f0

Thanks,

	tglx