On Fri, Dec 11, 2015 at 04:28:54PM +0100, Peter Zijlstra wrote:
> On Fri, Dec 11, 2015 at 03:36:36PM +0200, Alexander Shishkin wrote:
>
> > @@ -9063,6 +9621,18 @@ inherit_event(struct perf_event *parent_event,
> > get_ctx(child_ctx);
> >
> > /*
> > + * Clone itrace filters from the parent, if any
> > + */
> > + if (has_itrace_filter(child_event)) {
> > + if (perf_itrace_filters_clone(child_event, parent_event,
> > + child)) {
> > + put_ctx(child_ctx);
> > + free_event(child_event);
> > + return NULL;
>
> So inherit_event()'s return policy is somewhat opaque, there's 3
> possible returns:
>
> 1) a valid struct perf_event pointer; the clone was successful
> 2) ERR_PTR(err), the clone failed, abort inherit_group, fail fork()
> 3) NULL, the clone failed, ignore, continue
>
> We return NULL under two special cases:
>
> - the original event doesn't exist anymore, we're an orphan, do not make
> more orphans.
>
> - the parent event is dying
>
>
> I'm fairly sure this return should be in the 2) category. If we cannot
> fully clone the event something bad happened, we should not ignore it.
On second thought; we should not inherit the filters at all.
We should always use event->parent (if exists) for filters. Otherwise
inherited events will get different filters if you change the filter
after clone.
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