[PATCH 1/6] lib: Add two-byte cmpxchg emulation function

From: Bradley Morgan

Date: Fri Sep 11 2026 - 15:32:29 EST

cmpxchg_emu_u8() emulates one-byte cmpxchg() in terms of four-byte
cmpxchg() for the architectures lacking native one-byte atomics.
The same architectures also lack native two-byte cmpxchg(), where
such an operation is not supported and either fails to compile via
BUILD_BUG() or fails to link, because the bad pointer sentinels
these architectures declare are never defined.

Add cmpxchg_emu_u16(), the two-byte sibling. It reads the enclosing
word with READ_ONCE(), splices the two target bytes through a union
and loops on cmpxchg() of the full word until the compare succeeds.
Like cmpxchg_emu_u8() it is fully ordered.

The Kconfig symbol gating this file is renamed from
ARCH_NEED_CMPXCHG_1_EMU to ARCH_NEED_CMPXCHG_1_2_EMU, as it now
selects both the one-byte and the two-byte emulation.

Suggested-by: Paul E. McKenney <paulmck@xxxxxxxxxx>
Signed-off-by: Bradley Morgan <brads@xxxxxxxxxxxxxx>
---
arch/Kconfig | 2 +-
arch/arc/Kconfig | 2 +-
arch/arm/Kconfig | 2 +-
arch/csky/Kconfig | 2 +-
arch/sh/Kconfig | 2 +-
arch/xtensa/Kconfig | 2 +-
include/linux/cmpxchg-emu.h | 1 +
lib/Makefile | 2 +-
lib/cmpxchg-emu.c | 35 ++++++++++++++++++++++++++++++++---
9 files changed, 40 insertions(+), 10 deletions(-)

diff --git a/arch/Kconfig b/arch/Kconfig
index 3bb2e568f5b1..d56064797e09 100644
--- a/arch/Kconfig
+++ b/arch/Kconfig
@@ -1817,7 +1817,7 @@ config CC_HAS_SANE_FUNCTION_ALIGNMENT
# strict alignment always, even with -falign-functions.
def_bool CC_HAS_MIN_FUNCTION_ALIGNMENT || CC_IS_CLANG

-config ARCH_NEED_CMPXCHG_1_EMU
+config ARCH_NEED_CMPXCHG_1_2_EMU
bool

config ARCH_WANTS_PRE_LINK_VMLINUX
diff --git a/arch/arc/Kconfig b/arch/arc/Kconfig
index 80e61175bf52..0e91d254eccb 100644
--- a/arch/arc/Kconfig
+++ b/arch/arc/Kconfig
@@ -14,7 +14,7 @@ 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_NEED_CMPXCHG_1_2_EMU
select ARCH_SUPPORTS_ATOMIC_RMW if ARC_HAS_LLSC
select ARCH_32BIT_OFF_T
select BUILDTIME_TABLE_SORT
diff --git a/arch/arm/Kconfig b/arch/arm/Kconfig
index 408aa58a2a5b..8fef4238d411 100644
--- a/arch/arm/Kconfig
+++ b/arch/arm/Kconfig
@@ -38,7 +38,7 @@ 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_NEED_CMPXCHG_1_2_EMU if CPU_V6
select ARCH_SUPPORTS_ATOMIC_RMW
select ARCH_SUPPORTS_CFI
select ARCH_SUPPORTS_HUGETLBFS if ARM_LPAE
diff --git a/arch/csky/Kconfig b/arch/csky/Kconfig
index 4331313a42ff..5f26e8615d7d 100644
--- a/arch/csky/Kconfig
+++ b/arch/csky/Kconfig
@@ -37,7 +37,7 @@ config CSKY
select ARCH_INLINE_SPIN_UNLOCK_BH if !PREEMPTION
select ARCH_INLINE_SPIN_UNLOCK_IRQ if !PREEMPTION
select ARCH_INLINE_SPIN_UNLOCK_IRQRESTORE if !PREEMPTION
- select ARCH_NEED_CMPXCHG_1_EMU
+ select ARCH_NEED_CMPXCHG_1_2_EMU
select ARCH_WANT_FRAME_POINTERS if !CPU_CK610 && $(cc-option,-mbacktrace)
select ARCH_WANT_DEFAULT_TOPDOWN_MMAP_LAYOUT
select COMMON_CLK
diff --git a/arch/sh/Kconfig b/arch/sh/Kconfig
index d60f1d5a94c0..09e556fced01 100644
--- a/arch/sh/Kconfig
+++ b/arch/sh/Kconfig
@@ -14,7 +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 ARCH_NEED_CMPXCHG_1_2_EMU
select CPU_NO_EFFICIENT_FFS
select DMA_DECLARE_COHERENT
select GENERIC_ATOMIC64
diff --git a/arch/xtensa/Kconfig b/arch/xtensa/Kconfig
index f2f9cd9cde50..62019fd444d4 100644
--- a/arch/xtensa/Kconfig
+++ b/arch/xtensa/Kconfig
@@ -14,7 +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_NEED_CMPXCHG_1_2_EMU
select ARCH_USE_MEMTEST
select ARCH_USE_QUEUED_RWLOCKS
select ARCH_USE_QUEUED_SPINLOCKS
diff --git a/include/linux/cmpxchg-emu.h b/include/linux/cmpxchg-emu.h
index 998deec67740..fee8171fa05e 100644
--- a/include/linux/cmpxchg-emu.h
+++ b/include/linux/cmpxchg-emu.h
@@ -11,5 +11,6 @@
#define __LINUX_CMPXCHG_EMU_H

uintptr_t cmpxchg_emu_u8(volatile u8 *p, uintptr_t old, uintptr_t new);
+uintptr_t cmpxchg_emu_u16(volatile u16 *p, uintptr_t old, uintptr_t new);

#endif /* __LINUX_CMPXCHG_EMU_H */
diff --git a/lib/Makefile b/lib/Makefile
index 43421c39d21b..d0e69312176a 100644
--- a/lib/Makefile
+++ b/lib/Makefile
@@ -202,7 +202,7 @@ obj-$(CONFIG_CODE_TAGGING) += codetag.o
lib-$(CONFIG_GENERIC_BUG) += bug.o

obj-$(CONFIG_HAVE_ARCH_TRACEHOOK) += syscall.o
-obj-$(CONFIG_ARCH_NEED_CMPXCHG_1_EMU) += cmpxchg-emu.o
+obj-$(CONFIG_ARCH_NEED_CMPXCHG_1_2_EMU) += cmpxchg-emu.o

obj-$(CONFIG_DYNAMIC_DEBUG_CORE) += dynamic_debug.o
#ensure exported functions have prototypes
diff --git a/lib/cmpxchg-emu.c b/lib/cmpxchg-emu.c
index 27f6f97cb60d..991a5969f471 100644
--- a/lib/cmpxchg-emu.c
+++ b/lib/cmpxchg-emu.c
@@ -1,8 +1,8 @@
// SPDX-License-Identifier: GPL-2.0+
/*
- * Emulated 1-byte cmpxchg operation for architectures lacking direct
- * support for this size. This is implemented in terms of 4-byte cmpxchg
- * operations.
+ * Emulated 1-byte and 2-byte cmpxchg operations for architectures lacking
+ * direct support for these sizes. These are implemented in terms of
+ * 4-byte cmpxchg operations.
*
* Copyright (C) 2024 Paul E. McKenney.
*/
@@ -43,3 +43,32 @@ uintptr_t cmpxchg_emu_u8(volatile u8 *p, uintptr_t old, uintptr_t new)
return old;
}
EXPORT_SYMBOL_GPL(cmpxchg_emu_u8);
+
+union u16_32 {
+ u16 h[2];
+ u32 w;
+};
+
+/* Emulate two-byte cmpxchg() in terms of 4-byte cmpxchg. */
+uintptr_t cmpxchg_emu_u16(volatile u16 *p, uintptr_t old, uintptr_t new)
+{
+ u32 *p32 = (u32 *)(((uintptr_t)p) & ~0x3);
+ int i = (((uintptr_t)p) & 0x2) / 2;
+ union u16_32 old32;
+ union u16_32 new32;
+ u32 ret;
+
+ WARN_ON_ONCE(((uintptr_t)p) & 0x1);
+ ret = READ_ONCE(*p32);
+ do {
+ old32.w = ret;
+ if (old32.h[i] != old)
+ return old32.h[i];
+ new32.w = old32.w;
+ new32.h[i] = new;
+ instrument_atomic_read_write(p, 2);
+ ret = data_race(cmpxchg(p32, old32.w, new32.w)); // Overridden above.
+ } while (ret != old32.w);
+ return old;
+}
+EXPORT_SYMBOL_GPL(cmpxchg_emu_u16);
--
2.47.3