Re: kprobes and rcu_is_watching()

Ze Gao
2023-05-24T03:52:13Z

Thanks Steven, I think we've come to a consensus on this.

The question here is whether bpf tracing fentry i.e.,
__bpf_prog_enter{_sleepable}
needs to check rcu_is_watching as well before using rcu related
calls. And Yonghong suggested making a change when there is
indeed some bad case occurring since it's rare the tracee is in the idle path.

Regards,
Ze

On Tue, May 23, 2023 at 10:10 PM Steven Rostedt <rostedt@goodmis.org> wrote:
>
> [ Added a subject, as I always want to delete these emails as spam! ]
>
> On Mon, 22 May 2023 10:07:42 +0800
> Ze Gao <zegao2021@gmail.com> wrote:
>
> > Oops, I missed that. Thanks for pointing that out, which I thought is
> > conditional use of rcu_is_watching before.
> >
> > One last point, I think we should double check on this
> > "fentry does not filter with !rcu_is_watching"
> > as quoted from Yonghong and argue whether it needs
> > the same check for fentry as well.
> >
>
> Note that trace_test_and_set_recursion() (which is used by
> ftrace_test_recursion_trylock()) checks for rcu_is_watching() and
> returns false if it isn't (and the trylock will fail).
>
> -- Steve