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. ]
[ paulmck: Apply Linus Walleij feedback. ]

Reported-by: Mark Brown <[email protected]>
Closes: https://lore.kernel.org/all/[email protected]/
Reported-by: Naresh Kamboju <[email protected]>
Closes: https://lore.kernel.org/all/CA+G9fYuZ+pf6p8AXMZWtdFtX-gbG8HMaBKp=XbxcdzA_QeLkxQ@mail.gmail.com/
Signed-off-by: Paul E. McKenney <[email protected]>
Tested-by: Nathan Chancellor <[email protected]>
Reviewed-by: Linus Walleij <[email protected]>
Cc: "Russell King (Oracle)" <[email protected]>
Cc: Arnd Bergmann <[email protected]>
Cc: Andrew Davis <[email protected]>
Cc: Andrew Morton <[email protected]>
Cc: Eric DeVolder <[email protected]>
Cc: Rob Herring <[email protected]>
Cc: <[email protected]>

Author: paulmckrcu

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

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	/* ARCH == ARMv6 */
+	case 1:
+		oldval = cmpxchg_emu_u8((volatile u8 *)ptr, old, new);
+		break;
+#else /* min ARCH > ARMv6 */
 	case 1:
 		do {
 			asm volatile("@ __cmpxchg1\n"