console_cpu_notify can cause scheduling BUG during CPU hotplug

[Michael Bohan]

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 at codeaurora.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.
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