Re: workqueue: race in mod_delayed_work_on?

[Konstantin Khlebnikov <khlebnikov@yandex-team.ru>]

On 12.05.2016 16:06, Konstantin Khlebnikov wrote:
> On 10.05.2016 20:20, Konstantin Khlebnikov wrote:
>> On 10.05.2016 19:36, Tejun Heo wrote:
>>> Hello,
>>>
>>> On Tue, May 10, 2016 at 07:28:08PM +0300, Konstantin Khlebnikov wrote:
>>>> On 10.05.2016 11:21, Konstantin Khlebnikov wrote:
>>>>> I've got plenty warnings, bugs and oops around trivial use of mod_delayed_work in drivers/infiniband/core/addr.c
>>>>
>>>> Looks like problem in mod_delayed_work_on was hidden because add_timer is equal to mod_timer
>>>
>>> The timer usages are gated behind PENDING bit, so whether add_timer()
>>> is equal to mod_timer() shouldn't matter.
>>
>> Hmm... this looks little bit more complicated than one bit.
>
> Yep, problem was here - both timer and work can be active at the same time.

Nope, this is impossible. This will be a bug itself.

>
> So try_to_grab_pending can return success for two coRecentncurrent callers:
> first get del_timer, second removes work from workqueue. After that
> both call add timer and one of them either catch BUG_ON or corrupt timer list.
>
> I see two possible fixes: always remove timer and work in try_to_grab_pending
> but this must be carefully synchronized. This will make it slower for sure.
> Or always use mod_timer in  __queue_delayed_work() - both callers will modify timer,
> but here is no mod_timer_on().
>
>>
>>>
>>>> but Sasha accidentally backported 874bbfe600a660cba9c776b3957b1ce393151b76
>>>> (workqueue: make sure delayed work run in local cpu) into 3.18.25
>>>>
>>>> I don't see reason why that commit could break delayed work,
>>>> most likely it highlighted some other problem.
>>>
>>> What are you running?  Can you reproduce the issue on upstream kernel?
>>>
>>
>> This is slight patched 3.18.y. Looks like this started when we upgraded kernel to 3.18.25 and
>> somebody have loaded module ib_addr (ip in infiniband or something) which actually unused
>> because these machines have no infiniband at all. But this code is poked from ethernet arp
>> sometimes. So, it crashes somewhere from time to time. I'll try to stresstest this piece.
>>
>
>
>
>


-- 
Konstantin