perf_counter: Solve the rotate_ctx vs inherit race differently

Instead of disabling RR scheduling of the counters, use a different list
that does not get rotated to iterate the counters on inheritance.

[ Impact: cleanup, optimization ]
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Corey Ashford <cjashfor@linux.vnet.ibm.com>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Marcelo Tosatti <mtosatti@redhat.com>
Cc: John Kacur <jkacur@redhat.com>
LKML-Reference: <20090520102553.237504544@chello.nl>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

include/linux/perf_counter.h

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

kernel/perf_counter.c

@@ -1120,8 +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);
+	rotate_ctx(ctx);
 
 	perf_counter_cpu_sched_in(cpuctx, cpu);
 	perf_counter_task_sched_in(curr, cpu);
@@ -3109,7 +3108,6 @@ __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;
 }
 
@@ -3350,14 +3348,14 @@ 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:
 	 */
-	list_for_each_entry(counter, &parent_ctx->counter_list, list_entry) {
+	list_for_each_entry_rcu(counter, &parent_ctx->event_list, event_entry) {
+		if (counter != counter->group_leader)
+			continue;
+
 		if (!counter->hw_event.inherit)
 			continue;
 
@@ -3366,9 +3364,6 @@ void perf_counter_init_task(struct task_struct *child)
 			break;
 	}
 
-	barrier(); /* irqs */
-	parent_ctx->rr_allowed = 1;
-
 	mutex_unlock(&parent_ctx->mutex);
 }