Merge tag 'cmpxchg.2024.09.15a' of git://git.kernel.org/pub/scm/linux/kernel/git/paulmck/linux-rcu

Pull byte cmpxchg updates from Paul McKenney:
 "ARC/sh/xtensa: Provide one-byte cmpxchg emulation

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

  This covers all architectures"

* tag 'cmpxchg.2024.09.15a' of git://git.kernel.org/pub/scm/linux/kernel/git/paulmck/linux-rcu:
  xtensa: Emulate one-byte cmpxchg
  sh: Emulate one-byte cmpxchg
  ARC: Emulate one-byte cmpxchg

Author: torvalds

arch/arc/Kconfig

@@ -13,6 +13,7 @@ config ARC
 	select ARCH_HAS_SETUP_DMA_OPS
 	select ARCH_HAS_SYNC_DMA_FOR_CPU
 	select ARCH_HAS_SYNC_DMA_FOR_DEVICE
+	select ARCH_NEED_CMPXCHG_1_EMU
 	select ARCH_SUPPORTS_ATOMIC_RMW if ARC_HAS_LLSC
 	select ARCH_32BIT_OFF_T
 	select BUILDTIME_TABLE_SORT

arch/arc/include/asm/cmpxchg.h

@@ -8,6 +8,7 @@
 
 #include <linux/build_bug.h>
 #include <linux/types.h>
+#include <linux/cmpxchg-emu.h>
 
 #include <asm/barrier.h>
 #include <asm/smp.h>
@@ -46,6 +47,9 @@
 	__typeof__(*(ptr)) _prev_;					\
 									\
 	switch(sizeof((_p_))) {						\
+	case 1:								\
+		_prev_ = (__typeof__(*(ptr)))cmpxchg_emu_u8((volatile u8 *)_p_, (uintptr_t)_o_, (uintptr_t)_n_);	\
+		break;							\
 	case 4:								\
 		_prev_ = __cmpxchg(_p_, _o_, _n_);			\
 		break;							\
@@ -65,8 +69,6 @@
 	__typeof__(*(ptr)) _prev_;					\
 	unsigned long __flags;						\
 									\
-	BUILD_BUG_ON(sizeof(_p_) != 4);					\
-									\
 	/*								\
 	 * spin lock/unlock provide the needed smp_mb() before/after	\
 	 */								\

arch/sh/Kconfig

@@ -14,6 +14,7 @@ config SUPERH
 	select ARCH_HIBERNATION_POSSIBLE if MMU
 	select ARCH_MIGHT_HAVE_PC_PARPORT
 	select ARCH_WANT_IPC_PARSE_VERSION
+	select ARCH_NEED_CMPXCHG_1_EMU
 	select CPU_NO_EFFICIENT_FFS
 	select DMA_DECLARE_COHERENT
 	select GENERIC_ATOMIC64

arch/sh/include/asm/cmpxchg.h

@@ -9,6 +9,7 @@
 
 #include <linux/compiler.h>
 #include <linux/types.h>
+#include <linux/cmpxchg-emu.h>
 
 #if defined(CONFIG_GUSA_RB)
 #include <asm/cmpxchg-grb.h>
@@ -56,6 +57,8 @@ static inline unsigned long __cmpxchg(volatile void * ptr, unsigned long old,
 		unsigned long new, int size)
 {
 	switch (size) {
+	case 1:
+		return cmpxchg_emu_u8(ptr, old, new);
 	case 4:
 		return __cmpxchg_u32(ptr, old, new);
 	}

arch/xtensa/Kconfig

@@ -14,6 +14,7 @@ config XTENSA
 	select ARCH_HAS_DMA_SET_UNCACHED if MMU
 	select ARCH_HAS_STRNCPY_FROM_USER if !KASAN
 	select ARCH_HAS_STRNLEN_USER
+	select ARCH_NEED_CMPXCHG_1_EMU
 	select ARCH_USE_MEMTEST
 	select ARCH_USE_QUEUED_RWLOCKS
 	select ARCH_USE_QUEUED_SPINLOCKS

arch/xtensa/include/asm/cmpxchg.h

@@ -15,6 +15,7 @@
 
 #include <linux/bits.h>
 #include <linux/stringify.h>
+#include <linux/cmpxchg-emu.h>
 
 /*
  * cmpxchg
@@ -74,6 +75,7 @@ static __inline__ unsigned long
 __cmpxchg(volatile void *ptr, unsigned long old, unsigned long new, int size)
 {
 	switch (size) {
+	case 1:  return cmpxchg_emu_u8(ptr, old, new);
 	case 4:  return __cmpxchg_u32(ptr, old, new);
 	default: __cmpxchg_called_with_bad_pointer();
 		 return old;