Re: [PATCH] fprobe: Protect fprobe_return() with guard(rcu)()

[Masami Hiramatsu (Google) <mhiramat@kernel.org>]

On Tue, 29 Sep 2026 15:39:06 -0400
Steven Rostedt <rostedt@goodmis.org> wrote:

> On Mon, 28 Sep 2026 21:22:51 +0900
> Masami Hiramatsu (Google) <mhiramat@kernel.org> wrote:
> 
> > 
> > Hm, OK. Maybe we need to add something like checking rcu_watching, but
> > FTRACE_OPS_FL_RCU does not work correctly because fgraph_init_ops() only
> > configures subops (gops->ops). ftrace_startup_subops() does not propagete
> > that flag to graph_ops. (I guess that is yet another problem?)
> > 
> > To guarantee no illegal RCU usage or lockdep splats if an instrumented
> > function returns while RCU is offline, we can check 
> > 
> >  if (unlikely(!rcu_is_watching()))
> > 	return;
> 
> If rcu isn't watching then that is a bug.

Yeah, I made another fix for this bug.

https://lore.kernel.org/all/179064115227.394389.16910234241400391996.stgit@devnote2/

> 
> If you enable CONFIG_FTRACE_VALIDATE_RCU_IS_WATCHING then the
> ftrace_test_recursion_trylock() tests for rcu_is_watching(). We
> actually had that as default for some time to trigger any ftrace calls
> that were outside of RCU watching and was fixed.
> 
> Perhaps we need to enable that on more test machines to see if anything
> came back and allowed ftrace to trigger without it watching?

What about adding that config to tools/testing/selftests/ftrace/config?

Thank you,

> 
> -- Steve


-- 
Masami Hiramatsu (Google) <mhiramat@kernel.org>