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Re: net/bluetooth: workqueue destruction WARNING in hci_unregister_dev

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  Re: net/bluetooth: workqueue destruction WARNING in  hci_unregister_dev Tejun Heo <tj@kernel.org> - 2016-03-18 22:00 +0100
    Re: net/bluetooth: workqueue destruction WARNING in  hci_unregister_dev Jiri Slaby <jslaby@suse.cz> - 2016-03-21 17:00 +0100
      Re: net/bluetooth: workqueue destruction WARNING in  hci_unregister_dev Jiri Slaby <jslaby@suse.cz> - 2016-03-22 09:20 +0100
        Re: net/bluetooth: workqueue destruction WARNING in hci_unregister_dev Dmitry Vyukov <dvyukov@google.com> - 2016-03-22 13:40 +0100

#1360925 — Re: net/bluetooth: workqueue destruction WARNING in hci_unregister_dev

Hello, Jiri.

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.

Thanks.

-- 
tejun

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#1361953

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

This is what the code does now:
+               if ((pwq != wq->dfl_pwq) && (pwq->refcnt > 1)) {
+                       pr_info("%s: name='%s' pwq=%p wq->dfl_pwq=%p
pwq->refcnt=%d pwq->nr_active=%d delayed_works:",
+                                       __func__, wq->name, pwq,
wq->dfl_pwq,
+                                       pwq->refcnt, pwq->nr_active);
+
+                       show_pwq(pwq);
+
+                       mutex_unlock(&wq->mutex);
+                       show_state();
+                       show_workqueue_state();
+                       WARN_ON(1);
+                       return;
+               }

thanks,
-- 
js
suse labs

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#1362414

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.

PS. next on the table is the gsm tty warning.

thanks,
-- 
js
suse labs

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#1362698 — Re: net/bluetooth: workqueue destruction WARNING in hci_unregister_dev

On Tue, Mar 22, 2016 at 9:09 AM, Jiri Slaby <jslaby@suse.cz> 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.

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