Re: [PATCH] perf: Synchronously cleanup child events

[Peter Zijlstra]

On Mon, Jan 18, 2016 at 02:37:06PM +0200, Alexander Shishkin wrote:

> Or how about this instead:

In principle better since it doesn't increase the lock hold times etc.,
but buggy I think:

> diff --git a/kernel/events/core.c b/kernel/events/core.c
> index 8eb3fee429..cd9f1ac537 100644
> --- a/kernel/events/core.c
> +++ b/kernel/events/core.c
> @@ -3786,8 +3786,9 @@ int perf_event_release_kernel(struct perf_event *event)
>  
>  	event->owner = NULL;
>  
> +retry:
>  	/*
> -	 * event::child_mutex nests inside ctx::lock, so move children
> +	 * event::child_mutex nests inside ctx::mutex, so move children
>  	 * to a safe place first and avoid inversion
>  	 */
>  	mutex_lock(&event->child_mutex);
> @@ -3818,8 +3819,13 @@ int perf_event_release_kernel(struct perf_event *event)
>  		put_event(event);
>  	}
>  
> -	/* Must be the last reference */
> +	/* Must be the last reference, .. */
>  	put_event(event);
> +
> +	/* .. unless we raced with inherit_event(), in which case, repeat */

Nothing prevents @event from being freed here, which would make:

> +	if (atomic_long_inc_not_zero(&event->refcount))

a use-after-free.

You'll have to do something like:

static bool put_event_last(struct perf_event *event, long value)
{
	if (atomic_long_cmpxchg(&event->refcount, 1, 0)) {
		__put_event(event); /* normal put_event() body */
		return true;
	}
	return false;
}

with which you can do:

	if (!put_event_last(event))
		goto retry;