Re: [PATCH v7 00/16] tracing: probeevent: Improve fetcharg features

[Naveen N. Rao]

Masami Hiramatsu wrote:
> On Tue, 08 May 2018 15:41:11 +0530
> "Naveen N. Rao" <naveen.n.rao@xxxxxxxxxxxxxxxxxx> wrote:
>
> > Masami Hiramatsu wrote:
> > > On Mon, 07 May 2018 13:41:53 +0530
> > > "Naveen N. Rao" <naveen.n.rao@xxxxxxxxxxxxxxxxxx> wrote:
> > >> >> 
> > >> >> I didn't understand that. Which code are you planning to remove? Can you 
> > >> >> please elaborate? I thought we still need to disable preemption in the 
> > >> >> ftrace handler.
> > >> > 
> > >> > Yes, kprobe_ftrace_handler itself must be run under preempt disabled
> > >> > because it depends on a per-cpu variable. What I will remove is the
> > >> > redundant preempt disable/enable_noresched (unbalanced) pair in the
> > >> > kprobe_ftrace_handler, and jprobe x86 ports which is no more used.
> > >> 
> > >> Won't that break out-of-tree users depending on returning a non-zero 
> > >> value to handle preemption differently? You seem to have alluded to it 
> > >> earlier in the mail chain above where you said that this is not just for 
> > >> jprobes (though it was added for jprobes as the main use case).
> > > 
> > > No, all users are in tree already (function override for bpf and error-injection).
> >
> > Ok, so BPF error injection is a new user that can return a non-zero 
> > value from the pre handler. It looks like it can use KPROBES_ON_FTRACE 
> > too.
> >
> > In that case, on function entry, we call into kprobe_ftrace_handler() 
> > which will call fei_kprobe_handler(), which can re-enable premption 
> > before returning 1. So, if you remove the additional 
> > prempt_disable()/enable_no_resched() in kprobe_ftrace_handler(), then it 
> > will become imbalanced, right?
>
> Right. So we have to fix both at once. Please check the patch below.
>
> https://patchwork.kernel.org/patch/10386171/

Ah, so your intent was to change the semantics of how the pre handler 
works! I missed that aspect. This now makes sense. Thanks for the 
clarification.

- Naveen