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.
> +
> + 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.
