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

[Andrew Morton <akpm@linux-foundation.org>]

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.