Re: INFO: possible circular locking dependency at cleanup_workqueue_thread

[Ming Lei]

2009/5/19 Johannes Berg <johannes@xxxxxxxxxxxxxxxx>:
> On Tue, 2009-05-19 at 14:00 +0200, Oleg Nesterov wrote:
>
>> > I'm not familiar enough with the code -- but what are we really trying
>> > to do in CPU_POST_DEAD? It seems to me that at that time things must
>> > already be off the CPU, so ...?
>>
>> Yes, this cpu is dead, we should do cleanup_workqueue_thread() to kill
>> cwq->thread.
>>
>> > On the other hand that calls
>> > flush_cpu_workqueue() so it seems it would actually wait for the work to
>> > be executed on some other CPU, within the CPU_POST_DEAD notification?
>>
>> Yes. Because we can't just kill cwq->thread, we can have the pending
>> work_structs so we have to flush.
>>
>> Why can't we move these works to another CPU? We can, but this doesn't
>> really help. Because in any case we should at least wait for
>> cwq->current_work to complete.
>>
>> Why do we use CPU_POST_DEAD, and not (say) CPU_DEAD to flush/kill ?
>> Because work->func() can sleep in get_online_cpus(), we can't flush
>> until we drop cpu_hotplug.lock.
>
> Right. But exactly this happens in the hibernate case -- the hibernate
> code calls kernel/cpu.c:disable_nonboot_cpus() which calls _cpu_down()
> which calls raw_notifier_call_chain(&cpu_chain, CPU_POST_DEAD... Sadly,
> it does so while holding the cpu_add_remove_lock, which is happens to
> have the dependencies outlined in the original email...
>
> The same happens in cpu_down() (without leading _) which you can trigger
> from sysfs by manually removing the CPU, so it's not hibernate specific.
>
> Anyway, you can have a deadlock like this:
>
> CPU 3                   CPU 2                           CPU 1
>                                                        suspend/hibernate
>                        something:
>                        rtnl_lock()                     device_pm_lock()
>                                                        -> mutex_lock(&dpm_list_mtx)
>
>                        mutex_lock(&dpm_list_mtx)

Would you give a explaination why mutex_lock(&dpm_list_mtx) runs in CPU2
and depends on rtnl_lock?
Thanks!

>
> linkwatch_work
>  -> rtnl_lock()
>                                                        disable_nonboot_cpus()
>                                                        -> flush CPU 3 workqueue
>
> johannes
>
>



-- 
Lei Ming
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