x86, perfcounters: read out MSR_CORE_PERF_GLOBAL_STATUS with counters disabled

Impact: make perfcounter NMI and IRQ sequence more robust

Make __smp_perf_counter_interrupt() a bit more conservative: first disable
all counters, then read out the status. Most invocations are because there
are real events, so there's no performance impact.

Code flow gets a bit simpler as well this way.
Signed-off-by: Ingo Molnar <mingo@elte.hu>

arch/x86/kernel/cpu/perf_counter.c

@@ -383,18 +383,16 @@ static void __smp_perf_counter_interrupt(struct pt_regs *regs, int nmi)
 	struct cpu_hw_counters *cpuc;
 	u64 ack, status;
 
-	rdmsrl(MSR_CORE_PERF_GLOBAL_STATUS, status);
-	if (!status) {
-		ack_APIC_irq();
-		return;
-	}
-
 	/* Disable counters globally */
 	wrmsr(MSR_CORE_PERF_GLOBAL_CTRL, 0, 0);
 	ack_APIC_irq();
 
 	cpuc = &per_cpu(cpu_hw_counters, cpu);
 
+	rdmsrl(MSR_CORE_PERF_GLOBAL_STATUS, status);
+	if (!status)
+		goto out;
+
 again:
 	ack = status;
 	for_each_bit(bit, (unsigned long *) &status, nr_hw_counters) {
@@ -440,7 +438,7 @@ static void __smp_perf_counter_interrupt(struct pt_regs *regs, int nmi)
 	rdmsrl(MSR_CORE_PERF_GLOBAL_STATUS, status);
 	if (status)
 		goto again;
-
+out:
 	/*
 	 * Do not reenable when global enable is off:
 	 */