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
