Re: [RFC PATCH v2 26/31] fprobe: Rewrite fprobe on function-graph tracer

Masami Hiramatsu
2023-11-11T03:01:13Z

On Fri, 10 Nov 2023 20:44:22 -0500
Steven Rostedt <rostedt@goodmis.org> wrote:

> On Fri, 10 Nov 2023 16:17:39 +0900
> Masami Hiramatsu (Google) <mhiramat@kernel.org> wrote:
>
> > > + used = 0;
> > > + hlist_for_each_entry_from_rcu(node, hlist) {
> > > + if (node->addr != func)
> > > + break;
> > > + fp = READ_ONCE(node->fp);
> > > + if (!fp || fprobe_disabled(fp))
> > > + continue;
> > > +
> > > + if (fprobe_shared_with_kprobes(fp))
> > > + ret = __fprobe_kprobe_handler(func, ret_ip,
> > > + fp, fregs, fgraph_data + used);
> > > + else
> > > + ret = __fprobe_handler(func, ret_ip, fp,
> > > + fregs, fgraph_data + used);
> >
> >
> > Since the fgraph callback is under rcu-locked but not preempt-disabled,
>
> rcu-locked? The only rcu-locked is task rcu.

Hmm, it might be my misread. But I don't get any warning from
find_first_fprobe_node(), which uses hlist_for_each_entry_rcu()
so isn't it rcu locked?

>
> > fprobe unittest fails. I need to add preempt_disable_notrace() and
> > preempt_enable_notrace() around this. Note that kprobe_busy_begin()/end()
> > also access to per-cpu variable, so it requires to disable preemption.
>
>
> Just around the __fprobe_*handler()? Or the loop?

Just around the __fprobe*handler().

Thank you,

>
> -- Steve

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