Messages in this thread | | | Date | Fri, 20 Jul 2018 13:52:17 +0200 | From | Peter Zijlstra <> | Subject | Re: [PATCH v2] perf/core: fix a possible deadlock scenario |
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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?
> 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.
> Getting stuck in an hrtimer is a disaster:
You'll get NMI watchdog splats. Getting stuck in NMI context is far more 'interesting :-)
> +#define PERF_EF_NO_WAIT 0x08 /* do not wait when stopping, for > + * example, waiting for a timer > + */
That's a broken comment style.
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