Re: [PATCH -mm] cpuhotplug: introduce try_get_online_cpus() take 3

[Lai Jiangshan <laijs@cn.fujitsu.com>]

Andrew Morton wrote:
> On Tue, 09 Jun 2009 20:07:09 +0800
> Lai Jiangshan <laijs@cn.fujitsu.com> wrote:
> 
>> get_online_cpus() is a typically coarsely granular lock.
>> It's a source of ABBA deadlock.
>>
>> Thanks to the CPU notifiers, Some subsystem's global lock will
>> be required after cpu_hotplug.lock. Subsystem's global lock
>> is coarsely granular lock too, thus a lot's of lock in kernel
>> should be required after cpu_hotplug.lock(if we need
>> cpu_hotplug.lock held too)
>>
>> Otherwise it may come to a ABBA deadlock like this:
>>
>> thread 1                                      |        thread 2
>> _cpu_down()                                   |  Lock a-kernel-lock.
>>   cpu_hotplug_begin()                         |
>>     down_write(&cpu_hotplug.lock)             |
>>   __raw_notifier_call_chain(CPU_DOWN_PREPARE) |  get_online_cpus()
>> ------------------------------------------------------------------------
>>     Lock a-kernel-lock.(wait thread2)         |    down_read(&cpu_hotplug.lock)
>>                                                    (wait thread 1)
> 
> Confused.  cpu_hotplug_begin() doesn't do
> down_write(&cpu_hotplug.lock).  If it _were_ to do that then yes, we'd
> be vulnerable to the above deadlock.
> 

Ouch, this changelog is modified from the V1. But it not is modified
correctly. I apologize.

Lai.