linux-kernel - Re: [PATCH v2] perf: Synchronously cleanup child events

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Date:	Tue, 19 Jan 2016 21:05:58 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Alexander Shishkin <alexander.shishkin@...ux.intel.com>
Cc:	Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org,
	vince@...ter.net, eranian@...gle.com,
	Arnaldo Carvalho de Melo <acme@...radead.org>,
	Jiri Olsa <jolsa@...nel.org>, alexei.starovoitov@...il.com
Subject: Re: [PATCH v2] perf: Synchronously cleanup child events

On Tue, Jan 19, 2016 at 05:12:34PM +0200, Alexander Shishkin wrote:

> +static void __put_event(struct perf_event *event)
>  {
>  	struct perf_event_context *ctx;
>  
>  	if (!is_kernel_event(event))
>  		perf_remove_from_owner(event);
>  

> +int perf_event_release_kernel(struct perf_event *event)
>  {
> +	struct perf_event *child, *tmp;
> +	LIST_HEAD(child_list);
>  
> +	if (!is_kernel_event(event))
> +		perf_remove_from_owner(event);
>  
> +	event->owner = NULL;
>  
> +retry:

	<snip>

> +	/*
> +	 * If this is the last reference, we're done here, otherwise
> +	 * we must have raced with inherit_event(), in which case, repeat
> +	 */
> +	if (!put_event_last(event))
> +		goto retry;
>  
> +	return 0;
> +}

So I think there's a number of problems still :-(

I all starts with having two perf_remove_from_owner() calls (as I
mentioned on IRC), this doesn't make sense.

I think the moment you close the file and userspace looses control over
it, we should drop the owner bit, which is exactly the one
remove_from_owner in perf_release().

If, for some magical reason, the event lives on after that (and we'll
get to that), it should live on owner-less.

Now, assume someone has such a magical reference, then our
put_event_last() goto again loop will never terminate, this seems like a
bad thing.

The most obvious place that generates such magical references would be
the bpf arraymap doing perf_event_get() on things. There are a few other
places that take temp references (perf_mmap_close), but those are
'short' lived and while ugly will not cause massive grief. The BPF one
OTOH is a real problem here.

And looking at the BPF stuff, that code seems to assume
perf_event_kernel_release() := put_event(), so this patch breaks that
too.

Alexei, is there a reason the arraymap stuff needs a perf event ref as
opposed to a file ref? I'm forever a little confused on how perf<->bpf
works.