Re: [PATCH RFC v2 8/8] selftests/perf: Add kselftest for remove_on_exec

[Marco Elver]

On Tue, 23 Mar 2021 at 11:32, Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
>
> On Tue, Mar 23, 2021 at 10:52:41AM +0100, Marco Elver wrote:
>
> > with efs->func==__perf_event_enable. I believe it's sufficient to add
> >
> >       mutex_lock(&parent_event->child_mutex);
> >       list_del_init(&event->child_list);
> >       mutex_unlock(&parent_event->child_mutex);
> >
> > right before removing from context. With the version I have now (below
> > for completeness), extended torture with the above test results in no
> > more warnings and the test also passes.
> >
>
> > +     list_for_each_entry_safe(event, next, &ctx->event_list, event_entry) {
> > +             struct perf_event *parent_event = event->parent;
> > +
> > +             if (!event->attr.remove_on_exec)
> >                       continue;
> >
> > +             if (!is_kernel_event(event))
> > +                     perf_remove_from_owner(event);
> >
> > +             modified = true;
> > +
> > +             if (parent_event) {
> >                       /*
> > +                      * Remove event from parent, to avoid race where the
> > +                      * parent concurrently iterates through its children to
> > +                      * enable, disable, or otherwise modify an event.
> >                        */
> > +                     mutex_lock(&parent_event->child_mutex);
> > +                     list_del_init(&event->child_list);
> > +                     mutex_unlock(&parent_event->child_mutex);
> >               }
>
>                 ^^^ this, right?
>
> But that's something perf_event_exit_event() alread does. So then you're
> worried about the order of things.

Correct. We somehow need to prohibit the parent from doing an
event_function_call() while we potentially deactivate the context with
perf_remove_from_context().

> > +
> > +             perf_remove_from_context(event, !!event->parent * DETACH_GROUP);
> > +             perf_event_exit_event(event, ctx, current, true);
> >       }
>
> perf_event_release_kernel() first does perf_remove_from_context() and
> then clears the child_list, and that makes sense because if we're there,
> there's no external access anymore, the filedesc is gone and nobody will
> be iterating child_list anymore.
>
> perf_event_exit_task_context() and perf_event_exit_event() OTOH seem to
> rely on ctx->task == TOMBSTONE to sabotage event_function_call() such
> that if anybody is iterating the child_list, it'll NOP out.
>
> But here we don't have neither, and thus need to worry about the order
> vs child_list iteration.
>
> I suppose we should stick sync_child_event() in there as well.
>
> And at that point there's very little value in still using
> perf_event_exit_event()... let me see if there's something to be done
> about that.

I don't mind dropping use of perf_event_exit_event() and open coding
all of this. That would also avoid modifying perf_event_exit_event().

But I leave it to you what you think is nicest.

Thanks,
-- Marco