Re: net/bluetooth: workqueue destruction WARNING in hci_unregister_dev

[Dmitry Vyukov]

On Tue, Mar 22, 2016 at 9:09 AM, Jiri Slaby <jslaby@xxxxxxx> wrote:
> On 03/21/2016, 04:58 PM, Jiri Slaby wrote:
>> Hello,
>>
>> On 03/18/2016, 09:52 PM, Tejun Heo wrote:
>>> On Thu, Mar 17, 2016 at 01:00:13PM +0100, Jiri Slaby wrote:
>>>>>> I have not done that yet, but today, I see:
>>>>>> destroy_workqueue: name='req_hci0' pwq=ffff88002f590300
>>>>>> wq->dfl_pwq=ffff88002f591e00 pwq->refcnt=2 pwq->nr_active=0 delayed_works:
>>>>>>    pwq 12: cpus=0-1 node=0 flags=0x4 nice=-20 active=0/1
>>>>>>      in-flight: 18568:wq_barrier_func
>>>>>
>>>>> So, this means that there's flush_work() racing against workqueue
>>>>> destruction, which can't be safe. :(
>>>>
>>>> But I cannot trigger the WARN_ONs in the attached patch, so I am
>>>> confused how this can happen :(. (While I am still seeing the destroy
>>>> WARNINGs.)
>>>
>>> So, no operations should be in progress when destroy_workqueue() is
>>> called.  If somebody was flushing a work item, the flush call must
>>> have returned before destroy_workqueue() was invoked, which doesn't
>>> seem to be the case here.  Can you trigger BUG_ON() or sysrq-t when
>>> the above triggers?  There must be a task which is flushing a work
>>> item there and it shouldn't be difficult to pinpoint what's going on
>>> from it.
>>
>> The output of sysrq-t is here (> 200k), but I cannot see anything
>> suspicious in it:
>> http://www.fi.muni.cz/~xslaby/sklad/panics/jctl.txt
>
> Hmm, so I seem I cannot reproduce with this hunk:
> --- a/net/bluetooth/hci_core.c
> +++ b/net/bluetooth/hci_core.c
> @@ -3139,10 +3139,10 @@ void hci_unregister_dev(struct hci_dev *hdev)
>         list_del(&hdev->list);
>         write_unlock(&hci_dev_list_lock);
>
> -       hci_dev_do_close(hdev);
> -
>         cancel_work_sync(&hdev->power_on);
>
> +       hci_dev_do_close(hdev);
> +
>         if (!test_bit(HCI_INIT, &hdev->flags) &&
>             !hci_dev_test_flag(hdev, HCI_SETUP) &&
>             !hci_dev_test_flag(hdev, HCI_CONFIG)) {
>
>
>
> I cannot explain why though. I do not see how it matters in this
> particular case...
>
> Dmitry, could you apply it too? But I don't know how often you see the
> warning.

I've seen it only several times in several months, so I don't it will
be helpful.