Re:

Masami Hiramatsu
2023-05-22T01:37:01Z

On Sun, 21 May 2023 22:26:37 +0200
Jiri Olsa <olsajiri@gmail.com> wrote:

> On Sun, May 21, 2023 at 11:10:16PM +0800, Ze Gao wrote:
> > > kprobe_multi/fprobe share the same set of attachments with fentry.
> > > Currently, fentry does not filter with !rcu_is_watching, maybe
> > > because this is an extreme corner case. Not sure whether it is
> > > worthwhile or not.
> >
> > Agreed, it's rare, especially after Peter's patches which push narrow
> > down rcu eqs regions
> > in the idle path and reduce the chance of any traceable functions
> > happening in between.
> >
> > However, from RCU's perspective, we ought to check if rcu_is_watching
> > theoretically
> > when there's a chance our code will run in the idle path and also we
> > need rcu to be alive,
> > And also we cannot simply make assumptions for any future changes in
> > the idle path.
> > You know, just like what was hit in the thread.
> >
> > > Maybe if you can give a concrete example (e.g., attachment point)
> > > with current code base to show what the issue you encountered and
> > > it will make it easier to judge whether adding !rcu_is_watching()
> > > is necessary or not.
> >
> > I can reproduce likely warnings on v6.1.18 where arch_cpu_idle is
> > traceable but not on the latest version
> > so far. But as I state above, in theory we need it. So here is a
> > gentle ping :) .
>
> hum, this change [1] added rcu_is_watching check to ftrace_test_recursion_trylock,
> which we use in fprobe_handler and is coming to fprobe_exit_handler in [2]
>
> I might be missing something, but it seems like we don't need another
> rcu_is_watching call on kprobe_multi level

Good point! OK, then it seems we don't need it. The rethook continues to
use the rcu_is_watching() because it is also used from kprobes, but the
kprobe_multi doesn't need it.

Thank you,

>
> jirka
>
>
> [1] d099dbfd3306 cpuidle: tracing: Warn about !rcu_is_watching()
> [2] https://lore.kernel.org/bpf/20230517034510.15639-4-zegao@tencent.com/

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