Re: [RFC PATCH -tip 0/9]ftrace, kprobes: Ftrace-based kprobe optimization

[Masami Hiramatsu]

(2012/06/01 23:20), Steven Rostedt wrote:
> On Fri, 2012-06-01 at 22:36 +0900, Masami Hiramatsu wrote:
> 
>> OK, so I've introduced new noprobe tag and replaced __kprobes
>> with it. And now __kprobes tag which is a combination of noprobe
>> and notrace, means that the function is not probed and it can be
>> called from kprobe handler. (thus user must use this with their
>> handlers and functions which will be used from the handlers)
>> And also most of __kprobes tags are replaced by noprobe only.
> 
> You still haven't answered my question. Why can't function tracer still
> trace these? If kprobes does not allow it to be probed, it should not
> interfere with your code. But normal function tracing should still allow
> these.

Because those are called from ftrace-based kprobe, which means
it is directly invoked from kprobe_ftrace_handler. I think
that should be handled as a part of ftrace handler.
Currently, I just added notrace on below two kind of functions

 - handler functions which can be called intermediately from ftrace
 - get_kprobe, set_kprobe_instance, etc. internal utility functions
   which is called directly from kprobe ftrace handler.


> I still do not understand why you need to add 'notrace' at all.

Because I'd like to solve a recursive call problem.

I saw a problem which I hit some odd function tracer behavior.
When I removed notrace from get_kprobe(), which is an essential
internal function called directly from kprobe_ftrace_handler,
I hit a kernel crash caused by recursive call right after I
registered kprobe_ftrace_handler to ftrace. At that time,
ftrace_ops.filter was empty so I thought there is no function
traced, but the kprobe_ftrace_handler was called from somewhere.
So I saw it hit a recursive loop of ftrace_call ->
kprobe_ftrace_handler -> get_kprobe -> ftrace_call ...

I think if I just register kprobe's ftrace_ops without start
tracing, I think we can just do tracing without "notrace".

>> This means that you can trace those by function tracer :)
>>
>> BTW, currently kprobes allows user cases pagefault in their
>> handler (kprobe.fault_handler will handle it). I guess that
>> can cause some problem with ftrace, isn't it? If so, I need
>> to deny a kprobe using ftrace if it has fault_handler.
> 
> As long as there's recursion protection you are fine. In fact, I may add
> recursion protection within the assembly itself, that will make all
> function tracing safe. (does not solve the breakpoint bug from the other
> thread, but will solve most other things). In fact, this may allow us to
> remove notraces that were added because of recursion issues.

OK, I think kprobe already solves that as long as
get_kprobe and kprobe_running doesn't cause recursion...

Thank you,

-- 
Masami HIRAMATSU
Software Platform Research Dept. Linux Technology Center
Hitachi, Ltd., Yokohama Research Laboratory
E-mail: masami.hiramatsu.pt@xxxxxxxxxxx
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