On Sun, 14 Apr 2019 14:34:58 +0100 Russell King - ARM Linux admin <li...@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 <mhira...@kernel.org>