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

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Date:   Tue, 08 May 2018 15:41:11 +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 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?

> And also, for changing execution path by using kprobes, user handler must call
> not only preempt_enable(), but also clear current_kprobe per-cpu variable which
> is not exported to kmodules.

Ok, good point. And that means we don't have any external users any 
more.

Thanks,
Naveen