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    SubjectRe: Kconfig dependency issue on function-graph tracer and frame pointer on arm
    On Sun, 14 Apr 2019 14:34:58 +0100
    Russell King - ARM Linux admin <linux@armlinux.org.uk> wrote:

    > On Sun, Apr 14, 2019 at 07:47:05PM +0900, Masami Hiramatsu wrote:
    > > Hello,
    > >
    > > Recently, Naresh reported that the function-graph tracer on the latest
    > > kernel crashes on arm. I could reproduce it and bisected. I finally found
    > > the commit f9b58e8c7d03 ("ARM: 8800/1: use choice for kernel unwinders")
    > > was the first bad commit.
    >
    > I don't think that littering the rest of the kernel Kconfig with ARM
    > specific stuff is really a viable solution to this.
    >
    > If we examine the current situation, we have:
    >
    > - THUMB2_KERNEL selecting ARM_UNWIND when enabled.
    > - UNWINDER_FRAME_POINTER disabled if THUMB2_KERNEL is enabled, provided
    > we're not using Clang. This leaves UNWINDER_ARM as the only choice,
    > which also selects ARM_UNWIND.
    > - The default choice is dependent on the settings of AEABI and
    > FUNCTION_GRAPH_TRACER.
    > - HAVE_FUNCTION_GRAPH_TRACER is disabled if THUMB2_KERNEL is enabled.
    >
    > which seems to be _way_ too messy.
    >
    > Looking back before this commit, the function graph tracer never had a
    > strong dependence on frame pointers being enabled in the kernel, but it
    > seems the code relies upon them (see ftrace_return_to_handler() in
    > kernel/trace/ and return_to_handler in arch/arm/kernel/entry-frace.S).
    > There is also the __ftrace_graph_caller macro which seems to rely on it.

    Yes, so I think similar bug is hiding in other LTS kernels. It should
    have a dependency to FRAME_POINTER on arm.

    > Since Clang does not support frame pointers, we shouldn't even offer
    > the function graph tracer for Clang compilers, so let's do that with
    > the first hunk of the patch below.
    >
    > The subsequent hunks remove the defaulting of the choice according to
    > the function graph tracer - this is not a "hint" where the user can
    > still choose either option irrespective of the state of the function
    > graph tracer. They should only be able to select the frame pointer
    > option in that case.

    Agreed. Using default for making dependency is wrong.

    >
    > Another way forward would be for someone to put the work in to making
    > the function graph tracer work without frame pointers.

    Yes, we eventually need that. But for fixing current released kernel
    (this bug is in v5.0 series), I think Kconfig fix is needed.

    >
    > So, how about this:
    >
    > diff --git a/arch/arm/Kconfig b/arch/arm/Kconfig
    > index 850b4805e2d1..9aed25a6019b 100644
    > --- a/arch/arm/Kconfig
    > +++ b/arch/arm/Kconfig
    > @@ -73,7 +73,7 @@ config ARM
    > select HAVE_EFFICIENT_UNALIGNED_ACCESS if (CPU_V6 || CPU_V6K || CPU_V7) && MMU
    > select HAVE_EXIT_THREAD
    > select HAVE_FTRACE_MCOUNT_RECORD if !XIP_KERNEL
    > - select HAVE_FUNCTION_GRAPH_TRACER if !THUMB2_KERNEL
    > + select HAVE_FUNCTION_GRAPH_TRACER if !THUMB2_KERNEL && !CC_IS_CLANG
    > select HAVE_FUNCTION_TRACER if !XIP_KERNEL
    > select HAVE_GCC_PLUGINS
    > select HAVE_HW_BREAKPOINT if PERF_EVENTS && (CPU_V6 || CPU_V6K || CPU_V7)
    > diff --git a/arch/arm/Kconfig.debug b/arch/arm/Kconfig.debug
    > index 6d6e0330930b..e388af4594a6 100644
    > --- a/arch/arm/Kconfig.debug
    > +++ b/arch/arm/Kconfig.debug
    > @@ -47,8 +47,8 @@ config DEBUG_WX
    >
    > choice
    > prompt "Choose kernel unwinder"
    > - default UNWINDER_ARM if AEABI && !FUNCTION_GRAPH_TRACER
    > - default UNWINDER_FRAME_POINTER if !AEABI || FUNCTION_GRAPH_TRACER
    > + default UNWINDER_ARM if AEABI
    > + default UNWINDER_FRAME_POINTER if !AEABI

    If UNWINDER_ARM depends on ARM EABI, would we really need this "if !AEABI"?
    (I doubt we need these default...)

    > help
    > This determines which method will be used for unwinding kernel stack
    > traces for panics, oopses, bugs, warnings, perf, /proc/<pid>/stack,
    > @@ -65,7 +65,7 @@ config UNWINDER_FRAME_POINTER
    >
    > config UNWINDER_ARM
    > bool "ARM EABI stack unwinder"
    > - depends on AEABI
    > + depends on AEABI && !FUNCTION_GRAPH_TRACER

    Hmm, AFAIK, FUNCTION_GRAPH_TRACER only depends on FRAME_POINTER, but not
    UNWINDER_FRAME_POINTER. So can user still choose UNWINDER_ARM even if
    FUNCTION_GRAPH_TRACER=y ? (Of course that may not be a meaningful option)

    Thank you,

    > select ARM_UNWIND
    > help
    > This option enables stack unwinding support in the kernel
    >
    > --
    > RMK's Patch system: https://www.armlinux.org.uk/developer/patches/
    > FTTC broadband for 0.8mile line in suburbia: sync at 12.1Mbps down 622kbps up
    > According to speedtest.net: 11.9Mbps down 500kbps up


    --
    Masami Hiramatsu <mhiramat@kernel.org>

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