From mboxrd@z Thu Jan 1 00:00:00 1970 From: Vineet.Gupta1@synopsys.com (Vineet Gupta) Date: Tue, 17 Nov 2015 16:53:04 +0530 Subject: local64_cmpxchg() in arc_perf_event_update() In-Reply-To: <20151117110749.GT3816@twins.programming.kicks-ass.net> References: <1445286926.3913.13.camel@synopsys.com> <20151117110749.GT3816@twins.programming.kicks-ass.net> List-ID: Message-ID: <564B0E18.3040207@synopsys.com> To: linux-snps-arc@lists.infradead.org On Tuesday 17 November 2015 04:37 PM, Peter Zijlstra wrote: > On Tue, Nov 17, 2015@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);