Messages in this thread | | | Subject | Re: local64_cmpxchg() in arc_perf_event_update() | From | Vineet Gupta <> | Date | Tue, 17 Nov 2015 16:53:04 +0530 |
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On Tuesday 17 November 2015 04:37 PM, Peter Zijlstra wrote: > On Tue, Nov 17, 2015 at 09:14:59AM +0000, Vineet Gupta wrote: >> > Let's check with Peter as I'm not sure how exactly the read call will >> > nest for same counter on same core ? > Various possible ways, but the easiest is userspace doing a sys_read() > on the counter while the NMI happens. > >
That means Alexey need to revert the hunk ?
static void arc_perf_event_update(struct perf_event *event, struct hw_perf_event *hwc, int idx) { - uint64_t prev_raw_count, new_raw_count; - int64_t delta; - - do { - prev_raw_count = local64_read(&hwc->prev_count); - new_raw_count = arc_pmu_read_counter(idx); - } while (local64_cmpxchg(&hwc->prev_count, prev_raw_count, - new_raw_count) != prev_raw_count); - - delta = (new_raw_count - prev_raw_count) & - ((1ULL << arc_pmu->counter_size) - 1ULL); + uint64_t prev_raw_count = local64_read(&hwc->prev_count); + uint64_t new_raw_count = arc_pmu_read_counter(idx); + int64_t delta = new_raw_count - prev_raw_count;
+ /* + * We don't afaraid of hwc->prev_count changing beneath our feet + * because there's no way for us to re-enter this function anytime. + */ + local64_set(&hwc->prev_count, new_raw_count);
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