Groups | Search | Server Info | Keyboard shortcuts | Login | Register [http] [https] [nntp] [nntps]

Groups > linux.kernel > #1271090

Re: local64_cmpxchg() in arc_perf_event_update()

Show all headers | View raw

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);
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/

Back to linux.kernel | Previous | Next — Previous in thread | Next in thread | Find similar | Unroll thread

Thread

Re: local64_cmpxchg() in arc_perf_event_update() Vineet Gupta <Vineet.Gupta1@synopsys.com> - 2015-11-17 10:20 +0100
  Re: local64_cmpxchg() in arc_perf_event_update() Peter Zijlstra <peterz@infradead.org> - 2015-11-17 12:10 +0100
    Re: local64_cmpxchg() in arc_perf_event_update() Vineet Gupta <Vineet.Gupta1@synopsys.com> - 2015-11-17 12:30 +0100
      Re: local64_cmpxchg() in arc_perf_event_update() Peter Zijlstra <peterz@infradead.org> - 2015-11-17 13:30 +0100
        Re: local64_cmpxchg() in arc_perf_event_update() Peter Zijlstra <peterz@infradead.org> - 2015-11-17 13:30 +0100
          NMI for ARC Vineet Gupta <Vineet.Gupta1@synopsys.com> - 2015-11-17 14:00 +0100
            Re: NMI for ARC Peter Zijlstra <peterz@infradead.org> - 2015-11-17 14:20 +0100
          Re: local64_cmpxchg() in arc_perf_event_update() Vineet Gupta <Vineet.Gupta1@synopsys.com> - 2015-11-17 14:30 +0100

csiph-web