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

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Message-ID: <CAFTL4hw7+t8fzcphCyMAQoCBNb6RndQ7hiUnR6qhk6nYa+93fw@mail.gmail.com>
Date:	Tue, 25 Sep 2012 01:41:18 +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 Frederic Weisbecker <fweisbec@...il.com>:
> 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...

So either I should return immediately from rcu_user_exit() if
we are in an interrupt, or we make rcu_user_exit() able to nest
on rcu_irq_enter()   :)
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