Re: kernel deadlock

[John Stultz]

On 08/30/2013 04:04 PM, Gerlando Falauto wrote:
> Hi,
>
> sorry, it took me a while to narrow it down...
>
> On 08/30/2013 01:45 AM, John Stultz wrote:
>> On 08/29/2013 01:56 PM, Falauto, Gerlando wrote:
>>> Hi everyone,
>>>
>>> I ran into the deadlock situation reported at the bottom.
>>> Actually, on my latest 3.10 kernel for some reason I don't get the
>>> report (the kernel just hangs for some reason), so it took me quite
>>> some
>>> time to track it down.
>>>
>>> Once I figured the trigger to the machine hanging was adjtimex(), I
>>> reverted everything (between 3.9 to 3.10) that was touching
>>> kernel/time/timekeeping/timekeeping.c and kernel/time/ntp.c, I double
>>> checked that indeed the problem was not happening anymore, and finally
>>> started bisecting, landing on the following offending commit.
>>> THEN, and ONLY THEN, did I get the &%""ç+"% deadlock report.
>>>
>>> Do you guys have any ideas what could be wrong and how to fix it?
>>
>> Thanks for the report!
>>
>> What exactly is your process for reproducing the issue?
>
> Now (well, now...), it's quite easy.
>
> Three ingredients:
>
> 1) Kernel 3.10
>
> 2) Enable HRTICK
>
> diff --git a/kernel/sched/features.h b/kernel/sched/features.h
> index 99399f8..294e3ca 100644
> --- a/kernel/sched/features.h
> +++ b/kernel/sched/features.h
> @@ -41,7 +41,7 @@ SCHED_FEAT(WAKEUP_PREEMPTION, true)
>   */
>  SCHED_FEAT(ARCH_POWER, true)
>
> -SCHED_FEAT(HRTICK, false)
> +SCHED_FEAT(HRTICK, true)
>  SCHED_FEAT(DOUBLE_TICK, false)
>  SCHED_FEAT(LB_BIAS, true)
>
> 3) Run the following:
>
> #include <stdio.h>
> #include <sys/timex.h>
>
> int main(void)
> {
>     int i;
>
>     for (i = 0 ; ; i++) {
>     struct timex adj = {};
>     printf("%d\r", i);
>     fflush(stdout);
>     adjtimex(&adj);
>     }
>     return 0;
> }
>
> Notice how:
> 1) The original issue (with a bit more complicated scenario) was seen
> on ARM and PowerPC platforms
> 2) Under the above test conditions (on ARM) I *don't* get any deadlock
> report printed, the machine just hangs
> 3) The offending commit (below) I had found through a weird (manual)
> process of reverting and re-reverting (where some commits could have
> been reverted out of order), so I'm not 100% sure you'd come to the
> same conclusions.
>
> commit 06c017fdd4dc48451a29ac37fc1db4a3f86b7f40
> Author: John Stultz <john.stultz@xxxxxxxxxx>
> Date:   Fri Mar 22 11:37:28 2013 -0700
>
>      timekeeping: Hold timekeepering locks in do_adjtimex and hardpps
>
> I'm not able to perform any further testing at this very moment, but
> if needed, I can try bisecting again sometime next week, so to make an
> even more reliable statement.
>

Thanks so much for the details! I'll take a shot at reproducing this and
will let you know what comes of it.

thanks
-john

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