linux-kernel - Re: console_cpu_notify can cause scheduling BUG during CPU hotplug

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Message-ID: <4DB733D4.3000002@codeaurora.org>
Date:	Tue, 26 Apr 2011 14:06:28 -0700
From:	Michael Bohan <mbohan@...eaurora.org>
To:	Santosh Shilimkar <santosh.shilimkar@...com>
CC:	Kevin Cernekee <cernekee@...il.com>, mingo@...e.hu,
	akpm@...ux-foundation.org, simon.kagstrom@...insight.net,
	David.Woodhouse@...el.com, lethal@...ux-sh.org, tj@...nel.org,
	linux-kernel@...r.kernel.org, linux-arm-msm@...r.kernel.org,
	linux-arm-kernel@...ts.infradead.org
Subject: Re: console_cpu_notify can cause scheduling BUG during CPU hotplug

On 4/25/2011 10:58 PM, Santosh Shilimkar wrote:
> On 4/26/2011 5:48 AM, Kevin Cernekee wrote:
>> On Mon, Apr 25, 2011 at 4:33 PM, Michael Bohan<mbohan@...eaurora.org>
>> wrote:
>>> I was curious if this scenario was accounted for in the design of the
>>> console CPU notifier. One workaround for this problem is to remove
>>> CPU_DEAD
>>> from the possible actions in console_cpu_notify(). In fact, v1-v4 of the
>>> patch above did not have CPU_DEAD, CPU_DYING or CPU_DOWN_FAILED in
>>> the list
>>> of actions. I wasn't able to track down why the other cases were
>>> added in
>>> the final patch.
>>
>> Here is the background information on the CPU_{DEAD,DYING,DOWN_FAILED}
>> cases:
>>
>> http://lkml.org/lkml/2010/6/29/65
> That's right.
> May be the change log for commit '034260d67' would have been
> bit more descriptive about the CPU hot-plug events.

Thanks for the clarification. Now regarding the problem, it seems like 
we can't be taking a semaphore in that path. That is to say, we can't be 
calling console_lock from within stop_machine. A few options that come 
to mind:

-Use console_trylock and accept the possibility that the output is not 
guaranteed to be synchronous with the hotplug operation.
-Defer the console output emission (eg. workqueue) during hotplug.
-Hybrid of the two: if the console_trylock fails, then we defer the 
console output emission.

Any opinions? I can submit a patch if one of these approaches is reasonable.

Thanks,
Mike

-- 
Employee of Qualcomm Innovation Center, Inc.
Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum
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