Merge tag 'cmpxchg.2024.07.12a' of git://git.kernel.org/pub/scm/linux/kernel/git/paulmck/linux-rcu

Pull arm byte cmpxchg from Paul McKenney:
 "ARM: Provide one-byte cmpxchg emulation

  This provides emulated one-byte cmpxchg() support for ARM using the
  cmpxchg_emu_u8() function that uses a four-byte cmpxchg() to emulate
  the one-byte variant.

  Similar patches for emulation of one-byte cmpxchg() for arc, sh, and
  xtensa have not yet received maintainer acks, so they are slated for
  the v6.12 merge window"

* tag 'cmpxchg.2024.07.12a' of git://git.kernel.org/pub/scm/linux/kernel/git/paulmck/linux-rcu:
  ARM: Emulate one-byte cmpxchg

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"