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

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