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Date:	Wed, 24 Apr 2013 14:47:28 -0400
From:	Sasha Levin <sasha.levin@...cle.com>
To:	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...nel.org>
CC:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: lockdep: oops while dumping locks

Hi all,

I've noticed that sometimes lockdep would spew a warning and an oops
while dumping the list of held locks. I'm not quite sure why it happens
and I didn't find anything that was recently changed in the code that
could cause that:

[  855.807172] Showing all locks held in the system:
[  855.809162] 2 locks held by khungtaskd/3552:
[  855.809729]  #0:  (rcu_read_lock){.+.+..}, at: [<ffffffff811c3563>] watchdog+0x73/0x490
[  855.810956]  #1:  (tasklist_lock){.+.+..}, at: [<ffffffff8118c58c>] debug_show_all_locks+0x5c/0x260
[  855.815299] 1 lock held by ksmd/3578:
[  855.815770]  #0:  (ksm_thread_mutex){+.+.+.}, at: [<ffffffff8126653e>] ksm_scan_thread+0x4e/0x310
[  855.817055] 2 locks held by kpktgend_5/7480:
[  855.817597]  #0:  (rcu_read_lock){.+.+..}, at: [<          (null)>]           (null)
[  855.821794]  #1:
[  855.822044] ------------[ cut here ]------------
[  855.822677] WARNING: at kernel/lockdep.c:146 hlock_class+0x50/0x80()
[  855.823477] DEBUG_LOCKS_WARN_ON(1)
[  855.827953] Modules linked in:
[  855.828423] CPU: 1 PID: 3552 Comm: khungtaskd Tainted: G        W    3.9.0-rc8-next-20130423-sasha-00014-gdf37a98 #352
[  855.829749]  0000000000000009 ffff8800b8e05c28 ffffffff83fb6095 ffff8800b8e05c68
[  855.830798]  ffffffff811142db 00000000b58a3000 ffff8800adee0950 0000000000000002
[  855.835860]  0000000000000002 ffff8800b58a3000 ffff8800afa18000 ffff8800b8e05cc8
[  855.836887] Call Trace:
[  855.837222]  [<ffffffff83fb6095>] dump_stack+0x19/0x1b
[  855.840950]  [<ffffffff811142db>] warn_slowpath_common+0x6b/0xa0
[  855.841734]  [<ffffffff811143b1>] warn_slowpath_fmt+0x41/0x50
[  855.842472]  [<ffffffff83faa137>] ? printk+0x48/0x4a
[  855.843106]  [<ffffffff811881f0>] hlock_class+0x50/0x80
[  855.843758]  [<ffffffff83fab453>] print_lock+0xe/0x3d
[  855.847471]  [<ffffffff81189c92>] lockdep_print_held_locks+0xa2/0xe0
[  855.848561]  [<ffffffff8118c64a>] debug_show_all_locks+0x11a/0x260
[  855.849351]  [<ffffffff8115a427>] show_state_filter+0x207/0x220
[  855.850141]  [<ffffffff8115a220>] ? sched_show_task+0x270/0x270
[  855.853945]  [<ffffffff81189c92>] ? lockdep_print_held_locks+0xa2/0xe0
[  855.854756]  [<ffffffff8115a052>] ? sched_show_task+0xa2/0x270
[  855.855476]  [<ffffffff811c3803>] watchdog+0x313/0x490
[  855.856107]  [<ffffffff811c3563>] ? watchdog+0x73/0x490
[  855.856739]  [<ffffffff811c34f0>] ? hung_task_panic+0x20/0x20
[  855.862510]  [<ffffffff81146673>] kthread+0xe3/0xf0
[  855.863119]  [<ffffffff81146590>] ? kthread_create_on_node+0x160/0x160
[  855.863917]  [<ffffffff83fff1fc>] ret_from_fork+0x7c/0xb0
[  855.864575]  [<ffffffff81146590>] ? kthread_create_on_node+0x160/0x160
[  855.868426] ---[ end trace 3adc9bd194fdba9a ]---
[  855.869001] BUG: unable to handle kernel NULL pointer dereference at 0000000000000030
[  855.869983] IP: [<ffffffff81189a70>] __print_lock_name+0xc0/0xc0
[  855.870744] PGD 0
[  855.872483] Oops: 0000 [#1] PREEMPT SMP DEBUG_PAGEALLOC
[  855.872483] Dumping ftrace buffer:
[  855.872483]    (ftrace buffer empty)
[  855.872483] Modules linked in:
[  855.872483] CPU: 1 PID: 3552 Comm: khungtaskd Tainted: G        W    3.9.0-rc8-next-20130423-sasha-00014-gdf37a98 #352
[  855.872483] task: ffff8800b58a3000 ti: ffff8800b8e04000 task.ti: ffff8800b8e04000
[  855.872483] RIP: 0010:[<ffffffff81189a70>]  [<ffffffff81189a70>] __print_lock_name+0xc0/0xc0
[  855.872483] RSP: 0018:ffff8800b8e05cb0  EFLAGS: 00010296
[  855.872483] RAX: 0000000000000000 RBX: 0000000000000000 RCX: 000000008a728a71
[  855.872483] RDX: ffff8800b58a3000 RSI: ffff8800b8e05cc1 RDI: 0000000000000000
[  855.872483] RBP: ffff8800b8e05cd8 R08: 0000000000000002 R09: 0000000000000000
[  855.872483] R10: 0000000000000001 R11: 0000000000000000 R12: 0000000000000002
[  855.890101] R13: 0000000000000002 R14: ffff8800b58a3000 R15: ffff8800afa18000
[  855.891151] FS:  0000000000000000(0000) GS:ffff8800bb600000(0000) knlGS:0000000000000000
[  855.891151] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[  855.891151] CR2: 0000000000000030 CR3: 0000000005826000 CR4: 00000000000406e0
[  855.891151] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
[  855.891151] DR3: 0000000000000000 DR6: 00000000ffff0ff0 DR7: 0000000000000400
[  855.891151] Stack:
[  855.891151]  ffffffff81189bb5 0000000000000002 0000000000000002 ffff8800b8e05cd8
[  855.891151]  ffff8800adee0950 ffff8800b8e05cf8 ffffffff83fab45b 0000000000000038
[  855.891151]  ffff8800adee0000 ffff8800b8e05d28 ffffffff81189c92 ffff8800b8e05d28
[  855.891151] Call Trace:
[  855.891151]  [<ffffffff81189bb5>] ? print_lock_name+0x15/0x50
[  855.891151]  [<ffffffff83fab45b>] print_lock+0x16/0x3d
[  855.891151]  [<ffffffff81189c92>] lockdep_print_held_locks+0xa2/0xe0
[  855.891151]  [<ffffffff8118c64a>] debug_show_all_locks+0x11a/0x260
[  855.891151]  [<ffffffff8115a427>] show_state_filter+0x207/0x220
[  855.900353]  [<ffffffff8115a220>] ? sched_show_task+0x270/0x270
[  855.900353]  [<ffffffff81189c92>] ? lockdep_print_held_locks+0xa2/0xe0
[  855.900353]  [<ffffffff8115a052>] ? sched_show_task+0xa2/0x270
[  855.900353]  [<ffffffff811c3803>] watchdog+0x313/0x490
[  855.900353]  [<ffffffff811c3563>] ? watchdog+0x73/0x490
[  855.900353]  [<ffffffff811c34f0>] ? hung_task_panic+0x20/0x20
[  855.900353]  [<ffffffff81146673>] kthread+0xe3/0xf0
[  855.900353]  [<ffffffff81146590>] ? kthread_create_on_node+0x160/0x160
[  855.900353]  [<ffffffff83fff1fc>] ret_from_fork+0x7c/0xb0
[  855.900353]  [<ffffffff81146590>] ? kthread_create_on_node+0x160/0x160
[  855.900353] Code: 0f 1f 00 48 8b 45 e8 65 48 33 04 25 28 00 00 00 74 09 e8 d4 a6 f8 ff 0f 1f 40 00 48 81 c4 98 00 00 00 5b 5d
c3 66 0f 1f 44 00 00 <48> 8b 47 30 55 48 89 e5 a8 04 0f 85 a3 00 00 00 83 e0 01 f7 d8
[  855.900353] RIP  [<ffffffff81189a70>] __print_lock_name+0xc0/0xc0
[  855.900353]  RSP <ffff8800b8e05cb0>
[  855.900353] CR2: 0000000000000030
[  856.494698] ---[ end trace 3adc9bd194fdba9b ]---
--
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