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

[Jiri Olsa]

On Mon, Jul 16, 2018 at 02:51:01PM -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.
> 
> This kind of deadlock is already known and is catchable by lockdep,
> like for del_timer_sync(), we can add lockdep annotations. However,
> it is still missing for hrtimer_cancel(). (I have a WIP patch to make
> it complete for hrtimer_cancel() but it breaks booting.)
> 
> And there is such a deadlock scenario in kernel/events/core.c too,
> well actually, it is a simpler version: the hrtimer callback waits
> for itself to finish on the same CPU! It sounds stupid but it is
> not obvious at all, it hides very deeply in the perf event code:
> 
> 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()
> 
> Getting stuck in a timer doesn't sound very scary, however, in this

sound scary enough for me ;-) were you able to hit it?

> case, its consequences are a disaster:
> 
> perf_event_overflow() which calls __perf_event_overflow() is called
> in NMI handler too, so it is racy with hrtimer callback as disabling
> IRQ can't possibly disable NMI. This means this hrtimer callback
> once interrupted by an NMI handler could deadlock within NMI!

hum, the swevent pmu does not triger NMI, so that timer
will never be touched in NMI context

jirka