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.
