linux-kernel - Re: RCU idle CPU detection is broken in linux-next

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Date:	Tue, 25 Sep 2012 01:35:48 +0200
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Sasha Levin <levinsasha928@...il.com>
Cc:	paulmck@...ux.vnet.ibm.com,
	Michael Wang <wangyun@...ux.vnet.ibm.com>,
	Dave Jones <davej@...hat.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: RCU idle CPU detection is broken in linux-next

2012/9/25 Sasha Levin <levinsasha928@...il.com>:
> On 09/25/2012 01:06 AM, Frederic Weisbecker wrote:
>> 2012/9/25 Sasha Levin <levinsasha928@...il.com>:
>>> On 09/25/2012 12:47 AM, Sasha Levin wrote:
>>>>  - While I no longer see the warnings I've originally noticed, if I run with Paul's last debug patch I see the following warning:
>>>
>>> Correction: Original warnings are still there, they just got buried in the huge spew that was caused by additional debug warnings
>>> so I've missed them initially.
>>
>> Are they the same? Could you send me your dmesg?
>>
>> Thanks.
>>
>
> Log is attached, you can go directly to 168.703017 when the warnings begin.

Thanks!

So here is the first relevant warning:

[  168.703017] ------------[ cut here ]------------
[  168.708117] WARNING: at kernel/rcutree.c:502 rcu_eqs_exit_common+0x4a/0x3a0()
[  168.710034] Pid: 7871, comm: trinity-child65 Tainted: G        W
3.6.0-rc6-next-20120924-sasha-00030-g71f256c #5
[  168.710034] Call Trace:
[  168.710034]  <IRQ>  [<ffffffff811c737a>] ? rcu_eqs_exit_common+0x4a/0x3a0
[  168.710034]  [<ffffffff811078b6>] warn_slowpath_common+0x86/0xb0
[  168.710034]  [<ffffffff811079a5>] warn_slowpath_null+0x15/0x20
[  168.710034]  [<ffffffff811c737a>] rcu_eqs_exit_common+0x4a/0x3a0
[  168.710034]  [<ffffffff811c79cc>] rcu_eqs_exit+0x9c/0xb0
[  168.710034]  [<ffffffff811c7a4c>] rcu_user_exit+0x6c/0xd0
[  168.710034]  [<ffffffff8106eb1f>] do_general_protection+0x1f/0x170
[  168.710034]  [<ffffffff83a0e624>] ? restore_args+0x30/0x30
[  168.710034]  [<ffffffff83a0e875>] general_protection+0x25/0x30
[  168.710034]  [<ffffffff810a3f06>] ? native_read_msr_safe+0x6/0x20
[  168.710034]  [<ffffffff81a0b34b>] __rdmsr_safe_on_cpu+0x2b/0x50
[  168.710034]  [<ffffffff819ec971>] ? list_del+0x11/0x40
[  168.710034]  [<ffffffff811886dc>]
generic_smp_call_function_single_interrupt+0xec/0x120
[  168.710034]  [<ffffffff81151147>] ? account_system_vtime+0xd7/0x140
[  168.710034]  [<ffffffff81096f72>]
smp_call_function_single_interrupt+0x22/0x40
[  168.710034]  [<ffffffff83a0fe2f>] call_function_single_interrupt+0x6f/0x80
[  168.710034]  <EOI>  [<ffffffff83a0e5f4>] ? retint_restore_args+0x13/0x13
[  168.710034]  [<ffffffff811c7285>] ? rcu_user_enter+0x105/0x110
[  168.710034]  [<ffffffff8107e06d>] syscall_trace_leave+0xfd/0x150
[  168.710034]  [<ffffffff83a0f1ef>] int_check_syscall_exit_work+0x34/0x3d
[  168.710034] ---[ end trace fd408dd21b70b87c ]---

This is an exception inside an interrupt, and the interrupt
interrupted RCU user mode.
And we have that nesting:

rcu_irq_enter(); <--- irq entry
rcu_user_exit(); <--- exception entry

And rcu_eqs_exit() doesn't handle that very well...
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