perf_counter: fix counter inheritance race

Context rotation should not occur when we are in the middle of
walking the counter list when inheriting counters ...

[ Impact: fix occasionally incorrect perf stat results ]

Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Corey Ashford <cjashfor@linux.vnet.ibm.com>
Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Marcelo Tosatti <mtosatti@redhat.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

include/linux/perf_counter.h

@@ -508,6 +508,7 @@ struct perf_counter_context {
 	int			nr_counters;
 	int			nr_active;
 	int			is_active;
+	int			rr_allowed;
 	struct task_struct	*task;
 
 	/*

kernel/perf_counter.c

@@ -1120,6 +1120,7 @@ void perf_counter_task_tick(struct task_struct *curr, int cpu)
 	__perf_counter_task_sched_out(ctx);
 
 	rotate_ctx(&cpuctx->ctx);
+	if (ctx->rr_allowed)
 		rotate_ctx(ctx);
 
 	perf_counter_cpu_sched_in(cpuctx, cpu);
@@ -3108,6 +3109,7 @@ __perf_counter_init_context(struct perf_counter_context *ctx,
 	mutex_init(&ctx->mutex);
 	INIT_LIST_HEAD(&ctx->counter_list);
 	INIT_LIST_HEAD(&ctx->event_list);
+	ctx->rr_allowed = 1;
 	ctx->task = task;
 }
 
@@ -3348,6 +3350,9 @@ void perf_counter_init_task(struct task_struct *child)
 	 */
 	mutex_lock(&parent_ctx->mutex);
 
+	parent_ctx->rr_allowed = 0;
+	barrier(); /* irqs */
+
 	/*
 	 * We dont have to disable NMIs - we are only looking at
 	 * the list, not manipulating it:
@@ -3361,6 +3366,9 @@ void perf_counter_init_task(struct task_struct *child)
 			break;
 	}
 
+	barrier(); /* irqs */
+	parent_ctx->rr_allowed = 1;
+
 	mutex_unlock(&parent_ctx->mutex);
 }