arm-linux-gnueabihf-ld: kernel/rcu/update.o:update.c:(.text+0x1cc4): more undefined references to `__bad_cmpxchg' follow

[Nathan Chancellor]

On Thu, May 30, 2024 at 10:24:05AM -0700, Paul E. McKenney wrote:
> And for an untested first attempt at a fix.
> 
> What did I mess up this time?  ;-)

An include for cmpxchg-emu.h ;)

  In file included from arch/arm/include/asm/atomic.h:16,
                   from include/linux/atomic.h:7,
                   from include/asm-generic/bitops/lock.h:5,
                   from arch/arm/include/asm/bitops.h:245,
                   from include/linux/bitops.h:63,
                   from include/linux/log2.h:12,
                   from kernel/bounds.c:13:
  arch/arm/include/asm/cmpxchg.h: In function '__cmpxchg':
  arch/arm/include/asm/cmpxchg.h:167:26: error: implicit declaration of function 'cmpxchg_emu_u8' [-Werror=implicit-function-declaration]
    167 |                 oldval = cmpxchg_emu_u8((volatile u8 *)ptr, old, new);
        |                          ^~~~~~~~~~~~~~
  cc1: some warnings being treated as errors

> 							Thanx, Paul
> 
> ------------------------------------------------------------------------
> 
> commit 6e43483dd111cf1be58b02a45d0ca729ca2634ba
> Author: Paul E. McKenney <paulmck at kernel.org>
> Date:   Thu May 30 10:11:31 2024 -0700
> 
>     ARM: Emulate one-byte cmpxchg
>     
>     Use the new cmpxchg_emu_u8() to emulate one-byte cmpxchg() on ARM systems
>     with ARCH < ARMv6K.
>     
>     Reported-by: Mark Brown <broonie at kernel.org>
>     Closes: https://lore.kernel.org/all/54798f68-48f7-4c65-9cba-47c0bf175143@sirena.org.uk/
>     Reported-by: Naresh Kamboju <naresh.kamboju at linaro.org>
>     Closes: https://lore.kernel.org/all/CA+G9fYuZ+pf6p8AXMZWtdFtX-gbG8HMaBKp=XbxcdzA_QeLkxQ@mail.gmail.com/
>     Signed-off-by: Paul E. McKenney <paulmck at kernel.org>
>     Cc: "Russell King (Oracle)" <rmk+kernel at armlinux.org.uk>
>     Cc: Arnd Bergmann <arnd at arndb.de>
>     Cc: Andrew Davis <afd at ti.com>
>     Cc: Andrew Morton <akpm at linux-foundation.org>
>     Cc: Linus Walleij <linus.walleij at linaro.org>
>     Cc: Eric DeVolder <eric.devolder at oracle.com>
>     Cc: Rob Herring <robh at kernel.org>
>     Cc: <linux-arm-kernel at lists.infradead.org>
> 
> diff --git a/arch/arm/Kconfig b/arch/arm/Kconfig
> index ee5115252aac4..a867a7d967aa5 100644
> --- a/arch/arm/Kconfig
> +++ b/arch/arm/Kconfig
> @@ -34,6 +34,7 @@ config ARM
>  	select ARCH_MIGHT_HAVE_PC_PARPORT
>  	select ARCH_OPTIONAL_KERNEL_RWX if ARCH_HAS_STRICT_KERNEL_RWX
>  	select ARCH_OPTIONAL_KERNEL_RWX_DEFAULT if CPU_V7
> +	select ARCH_NEED_CMPXCHG_1_EMU if CPU_V6
>  	select ARCH_SUPPORTS_ATOMIC_RMW
>  	select ARCH_SUPPORTS_CFI_CLANG
>  	select ARCH_SUPPORTS_HUGETLBFS if ARM_LPAE
> diff --git a/arch/arm/include/asm/cmpxchg.h b/arch/arm/include/asm/cmpxchg.h
> index 44667bdb4707a..fd9b99f4dca46 100644
> --- a/arch/arm/include/asm/cmpxchg.h
> +++ b/arch/arm/include/asm/cmpxchg.h
> @@ -162,7 +162,11 @@ static inline unsigned long __cmpxchg(volatile void *ptr, unsigned long old,
>  	prefetchw((const void *)ptr);
>  
>  	switch (size) {
> -#ifndef CONFIG_CPU_V6	/* min ARCH >= ARMv6K */
> +#ifdef CONFIG_CPU_V6	/* min ARCH >= ARMv6K */
> +	case 1:
> +		oldval = cmpxchg_emu_u8((volatile u8 *)ptr, old, new);
> +		break;
> +#else
>  	case 1:
>  		do {
>  			asm volatile("@ __cmpxchg1\n"
> 
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