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    SubjectRe: [PATCH v2 3/9] rcu,tracing: Create trace_rcu_{enter,exit}()
    ----- On Feb 18, 2020, at 11:12 AM, paulmck paulmck@kernel.org wrote:

    > On Tue, Feb 18, 2020 at 01:33:35PM +0900, Masami Hiramatsu wrote:
    >> On Mon, 17 Feb 2020 08:31:12 -0800
    >> "Paul E. McKenney" <paulmck@kernel.org> wrote:
    >> >
    >> > > BTW, if you consider the x86 specific code is in the generic file,
    >> > > we can move NOKPROBE_SYMBOL() in arch/x86/kernel/traps.c.
    >> > > (Sorry, I've hit this idea right now)
    >> >
    >> > Might this affect other architectures with NMIs and probe-like things?
    >> > If so, it might make sense to leave it where it is.
    >>
    >> Yes, git grep shows that arm64 is using rcu_nmi_enter() in
    >> debug_exception_enter().
    >> OK, let's keep it, but maybe it is good to update the comment for
    >> arm64 too. What about following?
    >>
    >> +/*
    >> + * All functions in do_int3() on x86, do_debug_exception() on arm64 must be
    >> + * marked NOKPROBE before kprobes handler is called.
    >> + * ist_enter() on x86 and debug_exception_enter() on arm64 which is called
    >> + * before kprobes handle happens to call rcu_nmi_enter() which means
    >> + * that rcu_nmi_enter() must be marked NOKRPOBE.
    >> + */
    >
    > Would it work to describe the general problem, then give x86 details
    > as a specific example, as follows?
    >
    > /*
    > * On some architectures, certain exceptions prohibit use of kprobes until
    > * the exception code path reaches a certain point. For example, on x86 all
    > * functions called by do_int3() must be marked NOKPROBE. However, once
    > * kprobe_int3_handler() is called, kprobing is permitted. Specifically,
    > * ist_enter() is called in do_int3() before kprobe_int3_handle().
    > * Furthermore, ist_enter() calls rcu_nmi_enter(), which means that
    > * rcu_nmi_enter() must be marked NOKRPOBE.
    > */
    >
    > That way, I don't feel like I need to update the commment each time
    > a new architecture adds this capability. ;-)

    I suspect this kind of comment would belong in a new
    Documentation/features/kprobes/annotations.txt or something
    similar. You might want to look at other "features" files to see
    the expected layout.

    Thanks,

    Mathieu

    --
    Mathieu Desnoyers
    EfficiOS Inc.
    http://www.efficios.com

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