Messages in this thread | | | Date | Tue, 9 Jun 2009 12:34:38 -0700 | From | Andrew Morton <> | Subject | Re: [PATCH -mm] cpuhotplug: introduce try_get_online_cpus() take 3 |
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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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