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Message-Id: <1525802391.f0r6zmk1g8.naveen@linux.ibm.com>
Date: Tue, 08 May 2018 23:31:42 +0530
From: "Naveen N. Rao" <naveen.n.rao@...ux.vnet.ibm.com>
To: Masami Hiramatsu <mhiramat@...nel.org>
Cc: Arnaldo Carvalho de Melo <acme@...nel.org>,
Ananth N Mavinakayanahalli <ananth@...ux.vnet.ibm.com>,
linux-kernel@...r.kernel.org, linux-kselftest@...r.kernel.org,
linux-trace-users@...r.kernel.org, Ingo Molnar <mingo@...hat.com>,
Namhyung Kim <namhyung@...nel.org>,
Ravi Bangoria <ravi.bangoria@...ux.vnet.ibm.com>,
Steven Rostedt <rostedt@...dmis.org>, shuah@...nel.org,
Tom Zanussi <tom.zanussi@...ux.intel.com>
Subject: Re: [PATCH v7 00/16] tracing: probeevent: Improve fetcharg features
Masami Hiramatsu wrote:
> On Tue, 08 May 2018 15:41:11 +0530
> "Naveen N. Rao" <naveen.n.rao@...ux.vnet.ibm.com> wrote:
>
>> Masami Hiramatsu wrote:
>> > On Mon, 07 May 2018 13:41:53 +0530
>> > "Naveen N. Rao" <naveen.n.rao@...ux.vnet.ibm.com> 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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