Messages in this thread | | | Date | Wed, 10 Jun 2009 08:57:59 +0800 | From | Lai Jiangshan <> | Subject | Re: [PATCH -mm] cpuhotplug: introduce try_get_online_cpus() take 3 |
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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.
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