Re: [BUG] Deadlock due due to interactions of block, RCU, and cpu offline

[Jeffrey Hugo <jhugo@codeaurora.org>]

On 6/20/2017 5:46 PM, Paul E. McKenney wrote:
> On Mon, Mar 27, 2017 at 11:17:11AM -0700, Paul E. McKenney wrote:
>> On Mon, Mar 27, 2017 at 12:02:27PM -0600, Jeffrey Hugo wrote:
>>> Hi Paul.
>>>
>>> Thanks for the quick reply.
>>>
>>> On 3/26/2017 5:28 PM, Paul E. McKenney wrote:
>>>> On Sun, Mar 26, 2017 at 05:10:40PM -0600, Jeffrey Hugo wrote:
>>>
>>>>> It is a race between this work running, and the cpu offline processing.
>>>>
>>>> One quick way to test this assumption is to build a kernel with Kconfig
>>>> options CONFIG_RCU_NOCB_CPU=y and CONFIG_RCU_NOCB_CPU_ALL=y.  This will
>>>> cause call_rcu_sched() to queue the work to a kthread, which can migrate
>>>> to some other CPU.  If your analysis is correct, this should avoid
>>>> the deadlock.  (Note that the deadlock should be fixed in any case,
>>>> just a diagnostic assumption-check procedure.)
>>>
>>> I enabled CONFIG_RCU_EXPERT=y, CONFIG_RCU_NOCB_CPU=y,
>>> CONFIG_RCU_NOCB_CPU_ALL=y in my build.  I've only had time so far to
>>> do one test run however the issue reproduced, but it took a fair bit
>>> longer to do so.  An initial look at the data indicates that the
>>> work is still not running.  An odd observation, the two threads are
>>> no longer blocked on the same queue, but different ones.
>>
>> I was afraid of that...
>>
>>> Let me look at this more and see what is going on now.
>>
>> Another thing to try would be to affinity the "rcuo" kthreads to
>> some CPU that is never taken offline, just in case that kthread is
>> sometimes somehow getting stuck during the CPU-hotplug operation.
>>
>>>>> What is the opinion of the domain experts?
>>>>
>>>> I do hope that we can come up with a better fix.  No offense intended,
>>>> as coming up with -any- fix in the CPU-hotplug domain is not to be
>>>> denigrated, but this looks to be at vest quite fragile.
>>>>
>>>> 							Thanx, Paul
>>>>
>>>
>>> None taken.  I'm not particularly attached to the current fix.  I
>>> agree, it does appear to be quite fragile.
>>>
>>> I'm still not sure what a better solution would be though.  Maybe
>>> the RCU framework flushes the work somehow during cpu offline?  It
>>> would need to ensure further work is not queued after that point,
>>> which seems like it might be tricky to synchronize.  I don't know
>>> enough about the working of RCU to even attempt to implement that.
>>
>> There are some ways that RCU might be able to shrink the window during
>> which the outgoing CPU's callbacks are in limbo, but they are not free
>> of risk, so we really need to compleetly understand what is going on
>> before making any possibly ill-conceived changes.  ;-)
>>
>>> In any case, it seem like some more analysis is needed based on the
>>> latest data.
>>
>> Looking forward to hearing about you find!
> 
> Hearing nothing, I eventually took unilateral action (I am a citizen of
> USA, after all!) and produced the lightly tested patch shown below.
> 
> Does it help?
> 
> 							Thanx, Paul
> 

Wow, has it been 3 months already?  I am extremely sorry, I've been 
preempted multiple times, and this has sat on my todo list where I keep 
thinking I need to find time to come back to it but apparently not doing 
enough to make that happen.

Thank you for not forgetting about this.  I promise I will somehow clear 
my schedule to test this next week.

Thank you again.

-- 
Jeffrey Hugo
Qualcomm Datacenter Technologies as an affiliate of Qualcomm 
Technologies, Inc.
Qualcomm Technologies, Inc. is a member of the
Code Aurora Forum, a Linux Foundation Collaborative Project.