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

On Tue, 23 Mar 2021 at 11:32, Peter Zijlstra peterz@infradead.org 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