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
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