[lttng-dev] [RFC PATCH] timekeeping: introduce timekeeping_is_busy()

[John Stultz]

On 09/11/2013 10:49 AM, Mathieu Desnoyers wrote:
> Hi John,
>
> * John Stultz (john.stultz at linaro.org) wrote:
>> On 09/11/2013 08:08 AM, Mathieu Desnoyers wrote:
>>> Starting from commit 06c017fdd4dc48451a29ac37fc1db4a3f86b7f40
>>> "timekeeping: Hold timekeepering locks in do_adjtimex and hardpps"
>>> (3.10 kernels), the xtime write seqlock is held across calls to
>>> __do_adjtimex(), which includes a call to notify_cmos_timer(), and hence
>>> schedule_delayed_work().
>>>
>>> This introduces a side-effect for a set of tracepoints, including mainly
>>> the workqueue tracepoints: a tracer hooking on those tracepoints and
>>> reading current time with ktime_get() will cause hard system LOCKUP such
>>> as:
>> Oh bummer. I had just worked this issue out the other day:
>>     https://lkml.org/lkml/2013/9/9/476
>>
>> Apparently it was a schroedinbug of sorts. My apologies for time you
>> spent chasing this down.
> No worries. As soon as I've been able to reproduce it on my test box
> (with serial port), the NMI watchdog had a pretty reasonable explanation
> for the issue.
>
>> My plan is to pull the notify_cmos_timer call to outside of the
>> timekeeper locking (see the patch at the very end of the mail in the
>> above link), as well as try to add lockdep support to seqcount/seqlocks
>> so we can catch these sorts of issues more easily.
> I just tried your patch, and it indeed seems to fix the lockup I've been
> experiencing with lttng-modules. Do you plan pushing this fix into
> master, and submitting it for inclusion into stable 3.10 and stable 3.11 ?
Yea. I was waiting on feedback from the reporter that the fix resolves
the issue but if it fixes it for you I'll go ahead and send it out today.

thanks
-john