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



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