[PATCH 0/6] Add two-byte cmpxchg emulation and wire it into the architectures

[PATCH 0/6] Add two-byte cmpxchg emulation and wire it into the architectures

[Bradley Morgan]

Paul McKenney asked me off list to add cmpxchg_emu_u16() next to
cmpxchg_emu_u8() and wire it into the architectures that use the
one byte emulation, arc, ARMv6, csky, sh and xtensa.

Patch 1 adds the function, the prototype, and renames the Kconfig
symbol to ARCH_NEED_CMPXCHG_1_2_EMU since it now gates both
emulations. Patches 2 through 6 add case 2 beside each existing
case 1, following each file's current style. The function body
follows the two byte implementation in Paul's April 2024 RFC
series, of which only the one byte part landed as commit
a88d970c8bb5 ("lib: Add one-byte emulation function").

Per Paul's note, Arnd Bergmann's platform removal series was
checked before analyzing the users. It removes the imx31 and
omap24xx CPU_ARM1136R0 selectors, but INTEGRATOR_CM1136JFS still
selects CPU_ARM1136R0 until its scheduled removal in early 2027,
so the ARMv6 path in patch 3 is still live. The arc !LLSC path
already handles any size because it reads and writes through the
typed pointer under the atomic_ops lock, so only the LLSC switch
gains case 2.

Verification. The union splice is endian independent because h[i]
sits at byte offset 2*i on both layouts and i is (addr & 2) / 2,
so the selected halfword always matches the caller's byte offset.
A host test ran 240 cases across both halfword offsets, matching
and mismatching compares, against a byte level reference model,
all passing. The function also compiles big endian for ARM, which
matters because sh is a big endian user. lib/cmpxchg-emu.o builds
for arm with CONFIG_CPU_V6 and a test instantiation of cmpxchg()
on a u16 resolves to cmpxchg_emu_u16. The remaining checkpatch
warnings are the volatile and long line ones that the merged
cmpxchg_emu_u8() lines already carry.

Bradley Morgan (6):
  lib: Add two-byte cmpxchg emulation function
  ARC: Emulate two-byte cmpxchg
  ARM: Emulate two-byte cmpxchg on ARMv6
  csky: Emulate two-byte cmpxchg
  sh: Emulate two-byte cmpxchg
  xtensa: Emulate two-byte cmpxchg

 arch/Kconfig                      |  2 +-
 arch/arc/Kconfig                  |  2 +-
 arch/arc/include/asm/cmpxchg.h    |  3 +++
 arch/arm/Kconfig                  |  2 +-
 arch/arm/include/asm/cmpxchg.h    |  3 +++
 arch/csky/Kconfig                 |  2 +-
 arch/csky/include/asm/cmpxchg.h   |  9 ++++++++
 arch/sh/Kconfig                   |  2 +-
 arch/sh/include/asm/cmpxchg.h     |  2 ++
 arch/xtensa/Kconfig               |  2 +-
 arch/xtensa/include/asm/cmpxchg.h |  1 +
 include/linux/cmpxchg-emu.h       |  1 +
 lib/Makefile                      |  2 +-
 lib/cmpxchg-emu.c                 | 35 ++++++++++++++++++++++++++++---
 14 files changed, 58 insertions(+), 10 deletions(-)

-- 
2.47.3

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

[Bradley Morgan]

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@kernel.org>
Signed-off-by: Bradley Morgan <brads@mainlining.org>
---
 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

[PATCH 2/6] ARC: Emulate two-byte cmpxchg

[Bradley Morgan]

arc has no byte or halfword atomic memory operations, so the LLSC
arch_cmpxchg_relaxed() switch routes case 1 through cmpxchg_emu_u8()
and leaves case 2 to fall into the BUILD_BUG() default. Route case 2
through the new cmpxchg_emu_u16() instead.

The !CONFIG_ARC_HAS_LLSC arch_cmpxchg() already handles any size,
reading and writing through the typed pointer under the atomic_ops
lock, so it needs no change.

Signed-off-by: Bradley Morgan <brads@mainlining.org>
---
 arch/arc/include/asm/cmpxchg.h | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/arch/arc/include/asm/cmpxchg.h b/arch/arc/include/asm/cmpxchg.h
index 76f43db0890f..9b48b8fe0db2 100644
--- a/arch/arc/include/asm/cmpxchg.h
+++ b/arch/arc/include/asm/cmpxchg.h
@@ -50,6 +50,9 @@
 	case 1:								\
 		_prev_ = (__typeof__(*(ptr)))cmpxchg_emu_u8((volatile u8 *__force)_p_, (uintptr_t)_o_, (uintptr_t)_n_);	\
 		break;							\
+	case 2:								\
+		_prev_ = (__typeof__(*(ptr)))cmpxchg_emu_u16((volatile u16 *__force)_p_, (uintptr_t)_o_, (uintptr_t)_n_);	\
+		break;							\
 	case 4:								\
 		_prev_ = __cmpxchg(_p_, _o_, _n_);			\
 		break;							\
-- 
2.47.3

[PATCH 3/6] ARM: Emulate two-byte cmpxchg on ARMv6

[Bradley Morgan]

ARMv6 without the ARMv6K extensions, selected as CONFIG_CPU_V6,
has no ldrexh/strexh, so the __cmpxchg() switch only provides
case 1 through cmpxchg_emu_u8() and lets case 2 fall through to
__bad_cmpxchg(). Route case 2 through cmpxchg_emu_u16() so that
two-byte cmpxchg() works on these cores too.

ARMv6K and later already have native case 2 in the #else branch,
and __cmpxchg_local() already covers sizes 1 and 2 for ARMv6 via
__generic_cmpxchg_local(), so neither is touched.

Signed-off-by: Bradley Morgan <brads@mainlining.org>
---
 arch/arm/include/asm/cmpxchg.h | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/arch/arm/include/asm/cmpxchg.h b/arch/arm/include/asm/cmpxchg.h
index 9beb64d30586..6c0e2f21456b 100644
--- a/arch/arm/include/asm/cmpxchg.h
+++ b/arch/arm/include/asm/cmpxchg.h
@@ -167,6 +167,9 @@ static inline unsigned long __cmpxchg(volatile void *ptr, unsigned long old,
 	case 1:
 		oldval = cmpxchg_emu_u8((volatile u8 *)ptr, old, new);
 		break;
+	case 2:
+		oldval = cmpxchg_emu_u16((volatile u16 *)ptr, old, new);
+		break;
 #else /* min ARCH > ARMv6 */
 	case 1:
 		do {
-- 
2.47.3

[PATCH 4/6] csky: Emulate two-byte cmpxchg

[Bradley Morgan]

csky has no byte or halfword atomic memory operations, so the
__cmpxchg_relaxed(), __cmpxchg_acquire() and __cmpxchg() switches
each route case 1 through cmpxchg_emu_u8() and leave case 2 to
fall into the BUILD_BUG() default. Route case 2 in all three
through the new cmpxchg_emu_u16().

arch_cmpxchg_local() maps to __cmpxchg_relaxed() and so picks up
case 2 automatically.

Signed-off-by: Bradley Morgan <brads@mainlining.org>
---
 arch/csky/include/asm/cmpxchg.h | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/arch/csky/include/asm/cmpxchg.h b/arch/csky/include/asm/cmpxchg.h
index db6dda47184e..a6a4805064a4 100644
--- a/arch/csky/include/asm/cmpxchg.h
+++ b/arch/csky/include/asm/cmpxchg.h
@@ -65,6 +65,9 @@
 	case 1:							\
 		__ret = (__typeof__(*(ptr)))cmpxchg_emu_u8((volatile u8 *)__ptr, (uintptr_t)__old, (uintptr_t)__new); \
 		break;						\
+	case 2:							\
+		__ret = (__typeof__(*(ptr)))cmpxchg_emu_u16((volatile u16 *)__ptr, (uintptr_t)__old, (uintptr_t)__new); \
+		break;						\
 	case 4:							\
 		asm volatile (					\
 		"1:	ldex.w		%0, (%3) \n"		\
@@ -98,6 +101,9 @@
 	case 1:							\
 		__ret = (__typeof__(*(ptr)))cmpxchg_emu_u8((volatile u8 *)__ptr, (uintptr_t)__old, (uintptr_t)__new); \
 		break;						\
+	case 2:							\
+		__ret = (__typeof__(*(ptr)))cmpxchg_emu_u16((volatile u16 *)__ptr, (uintptr_t)__old, (uintptr_t)__new); \
+		break;						\
 	case 4:							\
 		asm volatile (					\
 		"1:	ldex.w		%0, (%3) \n"		\
@@ -132,6 +138,9 @@
 	case 1:							\
 		__ret = (__typeof__(*(ptr)))cmpxchg_emu_u8((volatile u8 *)__ptr, (uintptr_t)__old, (uintptr_t)__new); \
 		break;						\
+	case 2:							\
+		__ret = (__typeof__(*(ptr)))cmpxchg_emu_u16((volatile u16 *)__ptr, (uintptr_t)__old, (uintptr_t)__new); \
+		break;						\
 	case 4:							\
 		asm volatile (					\
 		RELEASE_FENCE					\
-- 
2.47.3

[PATCH 5/6] sh: Emulate two-byte cmpxchg

[Bradley Morgan]

SH has no byte or halfword atomic memory operations, so the
__cmpxchg() switch routes case 1 through cmpxchg_emu_u8() and
lets case 2 fall through to __cmpxchg_called_with_bad_pointer(),
which is declared but never defined, so a two-byte cmpxchg()
fails at link time. Route case 2 through the new cmpxchg_emu_u16().

Signed-off-by: Bradley Morgan <brads@mainlining.org>
---
 arch/sh/include/asm/cmpxchg.h | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/arch/sh/include/asm/cmpxchg.h b/arch/sh/include/asm/cmpxchg.h
index 1e5dc5ccf7bf..477d3025a441 100644
--- a/arch/sh/include/asm/cmpxchg.h
+++ b/arch/sh/include/asm/cmpxchg.h
@@ -59,6 +59,8 @@ static inline unsigned long __cmpxchg(volatile void * ptr, unsigned long old,
 	switch (size) {
 	case 1:
 		return cmpxchg_emu_u8(ptr, old, new);
+	case 2:
+		return cmpxchg_emu_u16(ptr, old, new);
 	case 4:
 		return __cmpxchg_u32(ptr, old, new);
 	}
-- 
2.47.3

[PATCH 6/6] xtensa: Emulate two-byte cmpxchg

[Bradley Morgan]

Xtensa has no byte or halfword atomic memory operations, so the
__cmpxchg() switch routes case 1 through cmpxchg_emu_u8() and
lets case 2 hit the default, __cmpxchg_called_with_bad_pointer(),
which is declared but never defined, so a two-byte cmpxchg() fails
at link time. Route case 2 through the new cmpxchg_emu_u16().

Signed-off-by: Bradley Morgan <brads@mainlining.org>
---
 arch/xtensa/include/asm/cmpxchg.h | 1 +
 1 file changed, 1 insertion(+)

diff --git a/arch/xtensa/include/asm/cmpxchg.h b/arch/xtensa/include/asm/cmpxchg.h
index b6db4838b175..8dea8e357fc0 100644
--- a/arch/xtensa/include/asm/cmpxchg.h
+++ b/arch/xtensa/include/asm/cmpxchg.h
@@ -76,6 +76,7 @@ __cmpxchg(volatile void *ptr, unsigned long old, unsigned long new, int size)
 {
 	switch (size) {
 	case 1:  return cmpxchg_emu_u8(ptr, old, new);
+	case 2:  return cmpxchg_emu_u16(ptr, old, new);
 	case 4:  return __cmpxchg_u32(ptr, old, new);
 	default: __cmpxchg_called_with_bad_pointer();
 		 return old;
-- 
2.47.3

Re: [PATCH 0/6] Add two-byte cmpxchg emulation and wire it into the architectures

[Arnd Bergmann]

On Fri, Sep 11, 2026, at 21:25, Bradley Morgan wrote:
> Per Paul's note, Arnd Bergmann's platform removal series was
> checked before analyzing the users. It removes the imx31 and
> omap24xx CPU_ARM1136R0 selectors, but INTEGRATOR_CM1136JFS still
> selects CPU_ARM1136R0 until its scheduled removal in early 2027,
> so the ARMv6 path in patch 3 is still live.

I can still restructure this and pull the INTEGRATOR_CM1136JFS
removal a little earlier. At the moment, this is part of the
patch to remove CPU_ARM1136R0, which I had planned to submit
once the arch/arm/mach-*/ platform removal patches are done:

https://git.kernel.org/pub/scm/linux/kernel/git/soc/soc.git/commit/?id=a8cfcc5533e33a

I could turn the mach-versatile/Kconfig change into a separate
(trivial) patch and add it to the platform series to avoid
churn here. The armv6 cmpxchg() code would still exist
in 7.4 then, but be unreachable.

       Arnd

Re: [PATCH 0/6] Add two-byte cmpxchg emulation and wire it into the architectures

[Bradley Morgan]

On 11 September 2026 21:17:15 BST, Arnd Bergmann <arnd@arndb.de> wrote:
>On Fri, Sep 11, 2026, at 21:25, Bradley Morgan wrote:
>> Per Paul's note, Arnd Bergmann's platform removal series was
>> checked before analyzing the users. It removes the imx31 and
>> omap24xx CPU_ARM1136R0 selectors, but INTEGRATOR_CM1136JFS still
>> selects CPU_ARM1136R0 until its scheduled removal in early 2027,
>> so the ARMv6 path in patch 3 is still live.
>
>I can still restructure this and pull the INTEGRATOR_CM1136JFS
>removal a little earlier. At the moment, this is part of the
>patch to remove CPU_ARM1136R0, which I had planned to submit
>once the arch/arm/mach-*/ platform removal patches are done:
>
>https://git.kernel.org/pub/scm/linux/kernel/git/soc/soc.git/commit/?id=a8cfcc5533e33a
>
>I could turn the mach-versatile/Kconfig change into a separate
>(trivial) patch and add it to the platform series to avoid
>churn here. The armv6 cmpxchg() code would still exist
>in 7.4 then, but be unreachable.
>
>       Arnd
Hi, is this like V2 required? Or a *ugh fine that's ok* kinda message?
--- Thanks!
https://lore.kernel.org/all/EE579805-42F2-4C58-B752-F28779EEB717@grrlz.net/

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

[David Laight]

On Fri, 11 Sep 2026 19:25:34 +0000
Bradley Morgan <brads@mainlining.org> wrote:

> 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@kernel.org>
> Signed-off-by: Bradley Morgan <brads@mainlining.org>
> ---
>  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/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);

Why uintptr_t? Shouldn't it just be u16?
(Which probably means the code would better if it was just 'unsigned int')

David

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

[Bradley Morgan]

On 11 September 2026 23:10:17 BST, David Laight
<david.laight.linux@gmail.com> wrote:
>On Fri, 11 Sep 2026 19:25:34 +0000
>Bradley Morgan <brads@mainlining.org> wrote:
>
>> 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@kernel.org>
>> Signed-off-by: Bradley Morgan <brads@mainlining.org>
>> ---
>>  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/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);
>
>Why uintptr_t? Shouldn't it just be u16?

Hi, Paul suggested I make it as identical as possible to u8, Do you reckon
this would break anything?


>(Which probably means the code would better if it was just 'unsigned int')

Ehh, subjective.

>
>David
>
>

--- Thanks!
https://lore.kernel.org/all/EE579805-42F2-4C58-B752-F28779EEB717@grrlz.net/

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

[Paul E. McKenney]

On Fri, Sep 11, 2026 at 11:10:17PM +0100, David Laight wrote:
> On Fri, 11 Sep 2026 19:25:34 +0000
> Bradley Morgan <brads@mainlining.org> wrote:
> 
> > 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@kernel.org>
> > Signed-off-by: Bradley Morgan <brads@mainlining.org>
> > ---
> >  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/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);
> 
> Why uintptr_t? Shouldn't it just be u16?
> (Which probably means the code would better if it was just 'unsigned int')

I suspect that Bradley is just following my cmpxchg_emu_u8() example,
which also returns uintptr_t.

I remember that *something* broke when I made this be u8, but I cannot
recall what the problem was.

Bradley, could you please try making it be u16 as David suggests just to
see what happens?  Who knows?  Maybe it was a compiler issue that has
since been fixed.  Or maybe the macros and asms using cmpxchg_emu_u8()
need that uintptr_t for some reason.

							Thanx, Paul

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

[David Laight]

On Fri, 11 Sep 2026 16:09:16 -0700
"Paul E. McKenney" <paulmck@kernel.org> wrote:

> On Fri, Sep 11, 2026 at 11:10:17PM +0100, David Laight wrote:
> > On Fri, 11 Sep 2026 19:25:34 +0000
> > Bradley Morgan <brads@mainlining.org> wrote:
> >   
> > > 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@kernel.org>
> > > Signed-off-by: Bradley Morgan <brads@mainlining.org>
> > > ---
> > >  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/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);  
> > 
> > Why uintptr_t? Shouldn't it just be u16?
> > (Which probably means the code would better if it was just 'unsigned int')  
> 
> I suspect that Bradley is just following my cmpxchg_emu_u8() example,
> which also returns uintptr_t.
> 
> I remember that *something* broke when I made this be u8, but I cannot
> recall what the problem was.
> 
> Bradley, could you please try making it be u16 as David suggests just to
> see what happens?  Who knows?  Maybe it was a compiler issue that has
> since been fixed.  Or maybe the macros and asms using cmpxchg_emu_u8()
> need that uintptr_t for some reason.

I think the uintptr (unsigned long) cast is needed to stop a compile
error when exchanging pointers.
But that is an issue with the #define not the called functions.

Possibly changing the #define to have:
	unsigned long ul_old = (unsigned long)(old);
Or even, with the type check from:
	unsigned long ul_old = (unsigned long)(0 ? *(ptr) : (old));
(with the same for 'new')
and the removing all the casts where the value are used might be better.

David

> 
> 							Thanx, Paul

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

[Bradley Morgan]

On 12 September 2026 10:30:47 BST, David Laight
<david.laight.linux@gmail.com> wrote:
>On Fri, 11 Sep 2026 16:09:16 -0700
>"Paul E. McKenney" <paulmck@kernel.org> wrote:
>
>> On Fri, Sep 11, 2026 at 11:10:17PM +0100, David Laight wrote:
>> > On Fri, 11 Sep 2026 19:25:34 +0000
>> > Bradley Morgan <brads@mainlining.org> wrote:
>> >   
>> > > 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@kernel.org>
>> > > Signed-off-by: Bradley Morgan <brads@mainlining.org>
>> > > ---
>> > >  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/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);  
>> > 
>> > Why uintptr_t? Shouldn't it just be u16?
>> > (Which probably means the code would better if it was just 'unsigned
>int')  
>> 
>> I suspect that Bradley is just following my cmpxchg_emu_u8() example,
>> which also returns uintptr_t.
>> 
>> I remember that *something* broke when I made this be u8, but I cannot
>> recall what the problem was.
>> 
>> Bradley, could you please try making it be u16 as David suggests just to
>> see what happens?  Who knows?  Maybe it was a compiler issue that has
>> since been fixed.  Or maybe the macros and asms using cmpxchg_emu_u8()
>> need that uintptr_t for some reason.
>
>I think the uintptr (unsigned long) cast is needed to stop a compile
>error when exchanging pointers.
>But that is an issue with the #define not the called functions.
>
>Possibly changing the #define to have:
>	unsigned long ul_old = (unsigned long)(old);
>Or even, with the type check from:
>	unsigned long ul_old = (unsigned long)(0 ? *(ptr) : (old));
>(with the same for 'new')
>and the removing all the casts where the value are used might be better.
>
>David
>
>> 
>> 							Thanx, Paul
>
>
Hmm. Okie dokie, noted for V2, thanks for the review, 

I'm sure arch code is just implementing this function, so I hope nothing
went wrong with the arch code.
--- Thanks!
https://lore.kernel.org/all/EE579805-42F2-4C58-B752-F28779EEB717@grrlz.net/

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

[Paul E. McKenney]

On Sat, Sep 12, 2026 at 10:30:47AM +0100, David Laight wrote:
> On Fri, 11 Sep 2026 16:09:16 -0700
> "Paul E. McKenney" <paulmck@kernel.org> wrote:
> 
> > On Fri, Sep 11, 2026 at 11:10:17PM +0100, David Laight wrote:
> > > On Fri, 11 Sep 2026 19:25:34 +0000
> > > Bradley Morgan <brads@mainlining.org> wrote:
> > >   
> > > > 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@kernel.org>
> > > > Signed-off-by: Bradley Morgan <brads@mainlining.org>
> > > > ---
> > > >  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/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);  
> > > 
> > > Why uintptr_t? Shouldn't it just be u16?
> > > (Which probably means the code would better if it was just 'unsigned int')  
> > 
> > I suspect that Bradley is just following my cmpxchg_emu_u8() example,
> > which also returns uintptr_t.
> > 
> > I remember that *something* broke when I made this be u8, but I cannot
> > recall what the problem was.
> > 
> > Bradley, could you please try making it be u16 as David suggests just to
> > see what happens?  Who knows?  Maybe it was a compiler issue that has
> > since been fixed.  Or maybe the macros and asms using cmpxchg_emu_u8()
> > need that uintptr_t for some reason.
> 
> I think the uintptr (unsigned long) cast is needed to stop a compile
> error when exchanging pointers.
> But that is an issue with the #define not the called functions.
> 
> Possibly changing the #define to have:
> 	unsigned long ul_old = (unsigned long)(old);
> Or even, with the type check from:
> 	unsigned long ul_old = (unsigned long)(0 ? *(ptr) : (old));
> (with the same for 'new')
> and the removing all the casts where the value are used might be better.

It does sound worth a try, especially since we have the uintptr_t version
to fall back on should that fail, or in case it somehow makes things
more complicated.

But we have been living with uintptr_t for some years with the 8-bit
code, so this should be able to be a follow-on patch.

							Thanx, Paul

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

[Bradley Morgan]

On 12 September 2026 19:58:02 BST, "Paul E. McKenney" <paulmck@kernel.org>
wrote:
>On Sat, Sep 12, 2026 at 10:30:47AM +0100, David Laight wrote:
>> On Fri, 11 Sep 2026 16:09:16 -0700
>> "Paul E. McKenney" <paulmck@kernel.org> wrote:
>> 
>> > On Fri, Sep 11, 2026 at 11:10:17PM +0100, David Laight wrote:
>> > > On Fri, 11 Sep 2026 19:25:34 +0000
>> > > Bradley Morgan <brads@mainlining.org> wrote:
>> > >   
>> > > > 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@kernel.org>
>> > > > Signed-off-by: Bradley Morgan <brads@mainlining.org>
>> > > > ---
>> > > >  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/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);  
>> > > 
>> > > Why uintptr_t? Shouldn't it just be u16?
>> > > (Which probably means the code would better if it was just 'unsigned
>int')  
>> > 
>> > I suspect that Bradley is just following my cmpxchg_emu_u8() example,
>> > which also returns uintptr_t.
>> > 
>> > I remember that *something* broke when I made this be u8, but I cannot
>> > recall what the problem was.
>> > 
>> > Bradley, could you please try making it be u16 as David suggests just
>to
>> > see what happens?  Who knows?  Maybe it was a compiler issue that has
>> > since been fixed.  Or maybe the macros and asms using cmpxchg_emu_u8()
>> > need that uintptr_t for some reason.
>> 
>> I think the uintptr (unsigned long) cast is needed to stop a compile
>> error when exchanging pointers.
>> But that is an issue with the #define not the called functions.
>> 
>> Possibly changing the #define to have:
>> 	unsigned long ul_old = (unsigned long)(old);
>> Or even, with the type check from:
>> 	unsigned long ul_old = (unsigned long)(0 ? *(ptr) : (old));
>> (with the same for 'new')
>> and the removing all the casts where the value are used might be better.
>
>It does sound worth a try, especially since we have the uintptr_t version
>to fall back on should that fail, or in case it somehow makes things
>more complicated.
>
>But we have been living with uintptr_t for some years with the 8-bit
>code, so this should be able to be a follow-on patch.
>
>							Thanx, Paul
I'll try u16 and then add another patch tryna fix the issue.
--- Thanks!
https://lore.kernel.org/all/EE579805-42F2-4C58-B752-F28779EEB717@grrlz.net/