On Sat, Dec 12, 2020 at 9:01 PM Thomas Gleixner <tglx@xxxxxxxxxxxxx> wrote:
>
> On Sat, Dec 12 2020 at 13:26, Marco Elver wrote:
> > On Thu, Mar 07, 2019 at 10:14AM +0100, Arnd Bergmann wrote:
> >> -static void __init futex_detect_cmpxchg(void)
> >> +static noinline void futex_detect_cmpxchg(void)
> >> {
> >> #ifndef CONFIG_HAVE_FUTEX_CMPXCHG
> >> u32 curval;
> >
> > What ever happened to this patch?
>
> It obviously fell through the cracks.
>
> > I'm seeing this again with the attached config + next-20201211 (for
> > testing https://bugs.llvm.org/show_bug.cgi?id=48492). Had to apply this
> > patch to build the kernel.
>
> What really bothers me is to remove the __init from a function which is
> clearly only used during init. And looking deeper it's simply a hack.
>
> This function is only needed when an architecture has to runtime
> discover whether the CPU supports it or not. ARM has unconditional
> support for this, so the obvious thing to do is the below.
>
Ah perfect, that is clearly the right solution here.
> --- a/arch/arm/Kconfig
> +++ b/arch/arm/Kconfig
> @@ -86,6 +86,7 @@ config ARM
> select HAVE_FTRACE_MCOUNT_RECORD if !XIP_KERNEL
> select HAVE_FUNCTION_GRAPH_TRACER if !THUMB2_KERNEL && !CC_IS_CLANG
> select HAVE_FUNCTION_TRACER if !XIP_KERNEL
> + select HAVE_FUTEX_CMPXCHG if FUTEX
> select HAVE_GCC_PLUGINS
> select HAVE_HW_BREAKPOINT if PERF_EVENTS && (CPU_V6 || CPU_V6K || CPU_V7)
> select HAVE_IDE if PCI || ISA || PCMCIA
I had a look at what other architectures always implement
futex_atomic_cmpxchg_inatomic() or can use the asm-generic non-SMP version,
and I found that it's pretty much all of them, the odd ones being just sparc32
and csky, which use asm-generic/futex.h but do have an SMP option,
as well as xtensa
I would guess that for csky, this is a mistake, as the architecture is fairly
new and should be able to implement it. Not sure about sparc32.
Arnd
