perf: Fix event->ctx locking

There have been a few reported issues wrt. the lack of locking around
changing event->ctx. This patch tries to address those.

It avoids the whole rwsem thing; and while it appears to work, please
give it some thought in review.

What I did fail at is sensible runtime checks on the use of
event->ctx, the RCU use makes it very hard.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Arnaldo Carvalho de Melo <acme@kernel.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Link: http://lkml.kernel.org/r/20150123125834.209535886@infradead.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>
perf_event_create_kernel_counter(struct perf_event_attr *attr, int cpu,
				 struct task_struct *task,
				 perf_overflow_handler_t overflow_handler,
				 void *context)
{
	struct perf_event_context *ctx;
	struct perf_event *event;
	int err;

	/*
	 * Get the target context (task or percpu):
	 */

	event = perf_event_alloc(attr, cpu, task, NULL, NULL,
				 overflow_handler, context);
	if (IS_ERR(event)) {
		err = PTR_ERR(event);
		goto err;
	}

	/* Mark owner so we could distinguish it from user events. */
	event->owner = EVENT_OWNER_KERNEL;

	account_event(event);

	ctx = find_get_context(event->pmu, task, cpu);
	if (IS_ERR(ctx)) {
		err = PTR_ERR(ctx);
		goto err_free;
	}

	WARN_ON_ONCE(ctx->parent_ctx);
	mutex_lock(&ctx->mutex);
	perf_install_in_context(ctx, event, cpu);
	perf_unpin_context(ctx);
	mutex_unlock(&ctx->mutex);

	return event;

err_free:
	free_event(event);
err:
	return ERR_PTR(err);
}

perf_event_create_kernel_counter(struct perf_event_attr *attr, int cpu,
				 struct task_struct *task,
				 perf_overflow_handler_t overflow_handler,
				 void *context)
{
	struct perf_event_context *ctx;
	struct perf_event *event;
	int err;

	/*
	 * Get the target context (task or percpu):
	 */

	event = perf_event_alloc(attr, cpu, task, NULL, NULL,
				 overflow_handler, context);
	if (IS_ERR(event)) {
		err = PTR_ERR(event);
		goto err;
	}

	/* Mark owner so we could distinguish it from user events. */
	event->owner = EVENT_OWNER_KERNEL;

	account_event(event);

	ctx = find_get_context(event->pmu, task, cpu);
	if (IS_ERR(ctx)) {
		err = PTR_ERR(ctx);
		goto err_free;
	}

	WARN_ON_ONCE(ctx->parent_ctx);
	mutex_lock(&ctx->mutex);
	perf_install_in_context(ctx, event, cpu);
	perf_unpin_context(ctx);
	mutex_unlock(&ctx->mutex);

	return event;

err_free:
	free_event(event);
err:
	return ERR_PTR(err);
}
