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From: Bradley Morgan <brads@mainlining.org>
To: Andrew Morton <akpm@linux-foundation.org>,
	Vineet Gupta <vgupta@kernel.org>, Guo Ren <guoren@kernel.org>,
	Yoshinori Sato <ysato@users.sourceforge.jp>,
	Rich Felker <dalias@libc.org>, Chris Zankel <chris@zankel.net>,
	Max Filippov <jcmvbkbc@gmail.com>
Cc: Arnd Bergmann <arnd@arndb.de>,
	"Paul E . McKenney" <paulmck@kernel.org>,
	David Laight <david.laight.linux@gmail.com>,
	John Paul Adrian Glaubitz <glaubitz@physik.fu-berlin.de>,
	linux-snps-arc@lists.infradead.org, linux-csky@vger.kernel.org,
	linux-sh@vger.kernel.org, linux-kernel@vger.kernel.org,
	Bradley Morgan <brads@mainlining.org>
Subject: [PATCH v3 3/5] csky: Emulate two-byte cmpxchg
Date: Thu, 17 Sep 2026 16:38:28 +0000	[thread overview]
Message-ID: <20260917163830.3748-4-brads@mainlining.org> (raw)
In-Reply-To: <20260917163830.3748-1-brads@mainlining.org>

CSKY has no two-byte atomic compare and swap, so the __cmpxchg()
switches in cmpxchg.h let case 2 fall through to the undefined
__cmpxchg_called_with_bad_pointer(), failing at link time. Route case
2 through the new cmpxchg_emu_u16(), which takes the old and new
values as unsigned long and narrows them itself, so the (uintptr_t)
casts on __old and __new come off in all three switch instances.
The (u16) casts are gone for the same reason.

The __old and __new declarations were typed __typeof__(old) and
__typeof__(new), which skips the pointer-integer type check, so
cmpxchg(&p, 4, 5) compiled silently. Typing them through
(unsigned long)(0 ? *(ptr) : (old)) keeps the value conversion while
making the compiler reject mismatched types, the idiom David
Laight suggested. The same check is added for new.

Signed-off-by: Bradley Morgan <brads@mainlining.org>
---
 arch/csky/include/asm/cmpxchg.h | 33 +++++++++++++++++++++------------
 1 file changed, 21 insertions(+), 12 deletions(-)

diff --git a/arch/csky/include/asm/cmpxchg.h b/arch/csky/include/asm/cmpxchg.h
index db6dda47184e..29dc56e4b7f1 100644
--- a/arch/csky/include/asm/cmpxchg.h
+++ b/arch/csky/include/asm/cmpxchg.h
@@ -57,13 +57,16 @@
 #define __cmpxchg_relaxed(ptr, old, new, size)			\
 ({								\
 	__typeof__(ptr) __ptr = (ptr);				\
-	__typeof__(new) __new = (new);				\
-	__typeof__(new) __tmp;					\
-	__typeof__(old) __old = (old);				\
+	unsigned long __old = (unsigned long)(0 ? *(ptr) : (old));	\
+	unsigned long __new = (unsigned long)(0 ? *(ptr) : (new));	\
+	unsigned long __tmp;					\
 	__typeof__(*(ptr)) __ret;				\
 	switch (size) {						\
 	case 1:							\
-		__ret = (__typeof__(*(ptr)))cmpxchg_emu_u8((volatile u8 *)__ptr, (uintptr_t)__old, (uintptr_t)__new); \
+		__ret = (__typeof__(*(ptr)))cmpxchg_emu_u8((volatile u8 *)__ptr, __old, __new); \
+		break;						\
+	case 2:							\
+		__ret = (__typeof__(*(ptr)))cmpxchg_emu_u16((volatile u16 *)__ptr, __old, __new); \
 		break;						\
 	case 4:							\
 		asm volatile (					\
@@ -90,13 +93,16 @@
 #define __cmpxchg_acquire(ptr, old, new, size)			\
 ({								\
 	__typeof__(ptr) __ptr = (ptr);				\
-	__typeof__(new) __new = (new);				\
-	__typeof__(new) __tmp;					\
-	__typeof__(old) __old = (old);				\
+	unsigned long __old = (unsigned long)(0 ? *(ptr) : (old));	\
+	unsigned long __new = (unsigned long)(0 ? *(ptr) : (new));	\
+	unsigned long __tmp;					\
 	__typeof__(*(ptr)) __ret;				\
 	switch (size) {						\
 	case 1:							\
-		__ret = (__typeof__(*(ptr)))cmpxchg_emu_u8((volatile u8 *)__ptr, (uintptr_t)__old, (uintptr_t)__new); \
+		__ret = (__typeof__(*(ptr)))cmpxchg_emu_u8((volatile u8 *)__ptr, __old, __new); \
+		break;						\
+	case 2:							\
+		__ret = (__typeof__(*(ptr)))cmpxchg_emu_u16((volatile u16 *)__ptr, __old, __new); \
 		break;						\
 	case 4:							\
 		asm volatile (					\
@@ -124,13 +130,16 @@
 #define __cmpxchg(ptr, old, new, size)				\
 ({								\
 	__typeof__(ptr) __ptr = (ptr);				\
-	__typeof__(new) __new = (new);				\
-	__typeof__(new) __tmp;					\
-	__typeof__(old) __old = (old);				\
+	unsigned long __old = (unsigned long)(0 ? *(ptr) : (old));	\
+	unsigned long __new = (unsigned long)(0 ? *(ptr) : (new));	\
+	unsigned long __tmp;					\
 	__typeof__(*(ptr)) __ret;				\
 	switch (size) {						\
 	case 1:							\
-		__ret = (__typeof__(*(ptr)))cmpxchg_emu_u8((volatile u8 *)__ptr, (uintptr_t)__old, (uintptr_t)__new); \
+		__ret = (__typeof__(*(ptr)))cmpxchg_emu_u8((volatile u8 *)__ptr, __old, __new); \
+		break;						\
+	case 2:							\
+		__ret = (__typeof__(*(ptr)))cmpxchg_emu_u16((volatile u16 *)__ptr, __old, __new); \
 		break;						\
 	case 4:							\
 		asm volatile (					\
-- 
2.47.3


WARNING: multiple messages have this Message-ID (diff)
From: Bradley Morgan <brads@mainlining.org>
To: Andrew Morton <akpm@linux-foundation.org>,
	Vineet Gupta <vgupta@kernel.org>, Guo Ren <guoren@kernel.org>,
	Yoshinori Sato <ysato@users.sourceforge.jp>,
	Rich Felker <dalias@libc.org>, Chris Zankel <chris@zankel.net>,
	Max Filippov <jcmvbkbc@gmail.com>
Cc: Arnd Bergmann <arnd@arndb.de>,
	"Paul E . McKenney" <paulmck@kernel.org>,
	David Laight <david.laight.linux@gmail.com>,
	John Paul Adrian Glaubitz <glaubitz@physik.fu-berlin.de>,
	linux-snps-arc@lists.infradead.org, linux-csky@vger.kernel.org,
	linux-sh@vger.kernel.org, linux-kernel@vger.kernel.org,
	Bradley Morgan <brads@mainlining.org>
Subject: [PATCH v3 3/5] csky: Emulate two-byte cmpxchg
Date: Thu, 17 Sep 2026 16:38:28 +0000	[thread overview]
Message-ID: <20260917163830.3748-4-brads@mainlining.org> (raw)
In-Reply-To: <20260917163830.3748-1-brads@mainlining.org>

CSKY has no two-byte atomic compare and swap, so the __cmpxchg()
switches in cmpxchg.h let case 2 fall through to the undefined
__cmpxchg_called_with_bad_pointer(), failing at link time. Route case
2 through the new cmpxchg_emu_u16(), which takes the old and new
values as unsigned long and narrows them itself, so the (uintptr_t)
casts on __old and __new come off in all three switch instances.
The (u16) casts are gone for the same reason.

The __old and __new declarations were typed __typeof__(old) and
__typeof__(new), which skips the pointer-integer type check, so
cmpxchg(&p, 4, 5) compiled silently. Typing them through
(unsigned long)(0 ? *(ptr) : (old)) keeps the value conversion while
making the compiler reject mismatched types, the idiom David
Laight suggested. The same check is added for new.

Signed-off-by: Bradley Morgan <brads@mainlining.org>
---
 arch/csky/include/asm/cmpxchg.h | 33 +++++++++++++++++++++------------
 1 file changed, 21 insertions(+), 12 deletions(-)

diff --git a/arch/csky/include/asm/cmpxchg.h b/arch/csky/include/asm/cmpxchg.h
index db6dda47184e..29dc56e4b7f1 100644
--- a/arch/csky/include/asm/cmpxchg.h
+++ b/arch/csky/include/asm/cmpxchg.h
@@ -57,13 +57,16 @@
 #define __cmpxchg_relaxed(ptr, old, new, size)			\
 ({								\
 	__typeof__(ptr) __ptr = (ptr);				\
-	__typeof__(new) __new = (new);				\
-	__typeof__(new) __tmp;					\
-	__typeof__(old) __old = (old);				\
+	unsigned long __old = (unsigned long)(0 ? *(ptr) : (old));	\
+	unsigned long __new = (unsigned long)(0 ? *(ptr) : (new));	\
+	unsigned long __tmp;					\
 	__typeof__(*(ptr)) __ret;				\
 	switch (size) {						\
 	case 1:							\
-		__ret = (__typeof__(*(ptr)))cmpxchg_emu_u8((volatile u8 *)__ptr, (uintptr_t)__old, (uintptr_t)__new); \
+		__ret = (__typeof__(*(ptr)))cmpxchg_emu_u8((volatile u8 *)__ptr, __old, __new); \
+		break;						\
+	case 2:							\
+		__ret = (__typeof__(*(ptr)))cmpxchg_emu_u16((volatile u16 *)__ptr, __old, __new); \
 		break;						\
 	case 4:							\
 		asm volatile (					\
@@ -90,13 +93,16 @@
 #define __cmpxchg_acquire(ptr, old, new, size)			\
 ({								\
 	__typeof__(ptr) __ptr = (ptr);				\
-	__typeof__(new) __new = (new);				\
-	__typeof__(new) __tmp;					\
-	__typeof__(old) __old = (old);				\
+	unsigned long __old = (unsigned long)(0 ? *(ptr) : (old));	\
+	unsigned long __new = (unsigned long)(0 ? *(ptr) : (new));	\
+	unsigned long __tmp;					\
 	__typeof__(*(ptr)) __ret;				\
 	switch (size) {						\
 	case 1:							\
-		__ret = (__typeof__(*(ptr)))cmpxchg_emu_u8((volatile u8 *)__ptr, (uintptr_t)__old, (uintptr_t)__new); \
+		__ret = (__typeof__(*(ptr)))cmpxchg_emu_u8((volatile u8 *)__ptr, __old, __new); \
+		break;						\
+	case 2:							\
+		__ret = (__typeof__(*(ptr)))cmpxchg_emu_u16((volatile u16 *)__ptr, __old, __new); \
 		break;						\
 	case 4:							\
 		asm volatile (					\
@@ -124,13 +130,16 @@
 #define __cmpxchg(ptr, old, new, size)				\
 ({								\
 	__typeof__(ptr) __ptr = (ptr);				\
-	__typeof__(new) __new = (new);				\
-	__typeof__(new) __tmp;					\
-	__typeof__(old) __old = (old);				\
+	unsigned long __old = (unsigned long)(0 ? *(ptr) : (old));	\
+	unsigned long __new = (unsigned long)(0 ? *(ptr) : (new));	\
+	unsigned long __tmp;					\
 	__typeof__(*(ptr)) __ret;				\
 	switch (size) {						\
 	case 1:							\
-		__ret = (__typeof__(*(ptr)))cmpxchg_emu_u8((volatile u8 *)__ptr, (uintptr_t)__old, (uintptr_t)__new); \
+		__ret = (__typeof__(*(ptr)))cmpxchg_emu_u8((volatile u8 *)__ptr, __old, __new); \
+		break;						\
+	case 2:							\
+		__ret = (__typeof__(*(ptr)))cmpxchg_emu_u16((volatile u16 *)__ptr, __old, __new); \
 		break;						\
 	case 4:							\
 		asm volatile (					\
-- 
2.47.3

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  parent reply	other threads:[~2026-09-17 16:40 UTC|newest]

Thread overview: 18+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2026-09-17 16:38 [PATCH v3 0/5] Add two-byte cmpxchg emulation and wire it into the architectures Bradley Morgan
2026-09-17 16:38 ` Bradley Morgan
2026-09-17 16:38 ` [PATCH v3 1/5] lib: Add two-byte cmpxchg emulation function Bradley Morgan
2026-09-17 16:38   ` Bradley Morgan
2026-09-18  8:59   ` David Laight
2026-09-18  8:59     ` David Laight
2026-09-17 16:38 ` [PATCH v3 2/5] ARC: Emulate two-byte cmpxchg Bradley Morgan
2026-09-17 16:38   ` Bradley Morgan
2026-09-17 16:38 ` Bradley Morgan [this message]
2026-09-17 16:38   ` [PATCH v3 3/5] csky: " Bradley Morgan
2026-09-17 16:38 ` [PATCH v3 4/5] sh: " Bradley Morgan
2026-09-17 16:38   ` Bradley Morgan
2026-09-17 16:38 ` [PATCH v3 5/5] xtensa: " Bradley Morgan
2026-09-17 16:38   ` Bradley Morgan
2026-09-17 16:42   ` Bradley Morgan
2026-09-17 16:42     ` Bradley Morgan
2026-09-18  9:10 ` [PATCH v3 0/5] Add two-byte cmpxchg emulation and wire it into the architectures David Laight
2026-09-18  9:10   ` David Laight

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