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

[Paul E. McKenney]

On Thu, May 30, 2024 at 02:52:48PM -0700, Nathan Chancellor wrote:
> 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

Good catch, and thank you for testing this!  Does the updated version
shown below do better?

							Thanx, Paul

------------------------------------------------------------------------

commit 74e3470afacaa9d2f37db4773a5fef887ac4ef56
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.
    
    [ paulmck: Apply Arnd Bergmann and Nathan Chancellor feedback. ]
    
    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..a428e06fe94ee 100644
--- a/arch/arm/include/asm/cmpxchg.h
+++ b/arch/arm/include/asm/cmpxchg.h
@@ -5,6 +5,7 @@
 #include <linux/irqflags.h>
 #include <linux/prefetch.h>
 #include <asm/barrier.h>
+#include <linux/cmpxchg-emu.h>
 
 #if defined(CONFIG_CPU_SA1100) || defined(CONFIG_CPU_SA110)
 /*
@@ -162,7 +163,11 @@ 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 /* min ARCH >= ARMv6K */
 	case 1:
 		do {
 			asm volatile("@ __cmpxchg1\n"