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SubjectRe: [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.

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