Re: [PATCH v2] perf/core: fix a possible deadlock scenario

[Cong Wang]

On Fri, Jul 20, 2018 at 4:52 AM Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
>
> On Thu, Jul 19, 2018 at 12:12:53PM -0700, Cong Wang wrote:
> > hrtimer_cancel() busy-waits for the hrtimer callback to stop,
> > pretty much like del_timer_sync(). This creates a possible deadlock
> > scenario where we hold a spinlock before calling hrtimer_cancel()
> > while in trying to acquire the same spinlock in the callback.
>
> Has this actually been observed?


Without lockdep annotation, it is not easy to observe.


>
> > cpu_clock_event_init():
> >   perf_swevent_init_hrtimer():
> >     hwc->hrtimer.function = perf_swevent_hrtimer;
> >
> > perf_swevent_hrtimer():
> >   __perf_event_overflow():
> >     __perf_event_account_interrupt():
> >       perf_adjust_period():
> >         pmu->stop():
> >         cpu_clock_event_stop():
> >           perf_swevent_cancel():
> >             hrtimer_cancel()
>
> Please explain how a hrtimer event ever gets to perf_adjust_period().
> Last I checked perf_swevent_init_hrtimer() results in attr.freq=0.

Good point.

I thought attr.freq is specified by user-space, but seems
perf_swevent_init_hrtimer() clears it purposely and will not change
after initialization, interesting...


>
> > Getting stuck in an hrtimer is a disaster:
>
> You'll get NMI watchdog splats. Getting stuck in NMI context is far more
> 'interesting :-)

Yes, I did see a stack trace in perf_swevent_hrtimer() which led
me here. But I have to admit among those hundreds of soft lockup's,
I only saw one showing swevent hrtimer backtrace.

Previously I thought this is because of NMI handler race, but Jiri
pointed out the race doesn't exist.


>
> > +#define PERF_EF_NO_WAIT      0x08            /* do not wait when stopping, for
> > +                                      * example, waiting for a timer
> > +                                      */
>
> That's a broken comment style.

It is picked by checkpatch.pl, not me, I chose a different one and got
a complain. :)

Thanks!