Re: [PATCH 2/3] workqueue: not allow recursion run_workqueue

[Lai Jiangshan <laijs@cn.fujitsu.com>]

Oleg Nesterov wrote:
> On 02/05, Lai Jiangshan wrote:
>> DEADLOCK EXAMPLE for explain my above option:
>>
>> (work_func0() and work_func1() are work callback, and they
>> calls flush_workqueue())
>>
>> CPU#0					CPU#1
>> run_workqueue()                         run_workqueue()
>>   work_func0()                            work_func1()
>>     flush_workqueue()                       flush_workqueue()
>>       flush_cpu_workqueue(0)                  .
>>       flush_cpu_workqueue(cpu#1)              flush_cpu_workqueue(cpu#0)
>>         waiting work_func1() in cpu#1           waiting work_func0 in cpu#0
>>
>> DEADLOCK!
> 
> I am not sure. Note that when work_func0() calls run_workqueue(),
> it will clear cwq->current_work, so another flush_ on CPU#1 will
> not wait for work_func0, no?

cwq->current_work is changed only when
!list_empty(&cwq->worklist)
in run_workqueue().

so cwq->current_work may not be changed.

> 
> But anyway. Nobody argues, "if (cwq->thread == current) {...}" code in
> flush_cpu_workqueue() is bad and should die. Otrherwise, we should
> fix the lockdep warning ;)
> 
> The only problem: if we still have the users of this hack, they will
> deadlock. But perhaps it is time to fix them.
> 
> And, if it was not clear, I do agree with this change. And Peter
> seems to agree as well.
> 
> Oleg.
> 
> 
> 
>