[PATCH 2/5] ARC: Emulate two-byte cmpxchg

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 2/5] ARC: Emulate two-byte cmpxchg
Date: Tue, 22 Sep 2026 17:33:51 +0000	[thread overview]
Message-ID: <20260922173354.14404-3-brads@mainlining.org> (raw)
In-Reply-To: <20260922173354.14404-1-brads@mainlining.org>

ARC has no two-byte atomic compare and swap, so the arch_cmpxchg_relaxed()
macro switch lets case 2 fall through to BUILD_BUG() via default. 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 _o_ and _n_ are no longer needed and the case 2 call passes
them straight.

The switch also now tests sizeof(*(_p_)) instead of sizeof((_p_)),
which switched on the pointer and made the size 1 and size 2 cases
dead code, routing every sub-word cmpxchg() through the 32-bit
llock/scond pair and comparing whole words against sub-word values,
so the compare almost never succeeded. The old and new values are
now declared as unsigned long through (unsigned long)(0 ? *(_p_) :
(old)), the idiom David Laight suggested, which type checks the
arguments against the pointee, so cmpxchg(&p, 4, 5) no longer
compiles silently.

Signed-off-by: Bradley Morgan <brads@mainlining.org>
---
 arch/arc/include/asm/cmpxchg.h | 19 +++++++++++--------
 1 file changed, 11 insertions(+), 8 deletions(-)

diff --git a/arch/arc/include/asm/cmpxchg.h b/arch/arc/include/asm/cmpxchg.h
index 76f43db0890f..4596edfefa95 100644
--- a/arch/arc/include/asm/cmpxchg.h
+++ b/arch/arc/include/asm/cmpxchg.h
@@ -42,16 +42,19 @@
 #define arch_cmpxchg_relaxed(ptr, old, new)				\
 ({									\
 	__typeof__(ptr) _p_ = (ptr);					\
-	__typeof__(*(ptr)) _o_ = (old);					\
-	__typeof__(*(ptr)) _n_ = (new);					\
+	unsigned long _old_ = (unsigned long)(0 ? *(_p_) : (old));		\
+	unsigned long _new_ = (unsigned long)(0 ? *(_p_) : (new));		\
 	__typeof__(*(ptr)) _prev_;					\
 									\
-	switch(sizeof((_p_))) {						\
+	switch (sizeof(*(_p_))) {						\
 	case 1:								\
-		_prev_ = (__typeof__(*(ptr)))cmpxchg_emu_u8((volatile u8 *__force)_p_, (uintptr_t)_o_, (uintptr_t)_n_);	\
+		_prev_ = (__typeof__(*(ptr)))cmpxchg_emu_u8((volatile u8 *__force)_p_, _old_, _new_);	\
+		break;							\
+	case 2:								\
+		_prev_ = (__typeof__(*(ptr)))cmpxchg_emu_u16((volatile u16 *__force)_p_, _old_, _new_);	\
 		break;							\
 	case 4:								\
-		_prev_ = __cmpxchg(_p_, _o_, _n_);			\
+		_prev_ = (__typeof__(*(ptr)))__cmpxchg(_p_, _old_, _new_);			\
 		break;							\
 	default:							\
 		BUILD_BUG();						\
@@ -64,8 +67,8 @@
 #define arch_cmpxchg(ptr, old, new)				        \
 ({									\
 	volatile __typeof__(ptr) _p_ = (ptr);				\
-	__typeof__(*(ptr)) _o_ = (old);					\
-	__typeof__(*(ptr)) _n_ = (new);					\
+	unsigned long _old_ = (unsigned long)(0 ? *(_p_) : (old));		\
+	unsigned long _new_ = (unsigned long)(0 ? *(_p_) : (new));		\
 	__typeof__(*(ptr)) _prev_;					\
 	unsigned long __flags;						\
 									\
@@ -102,7 +105,7 @@
 	__typeof__(ptr) _p_ = (ptr);					\
 	__typeof__(*(ptr)) _val_ = (val);				\
 									\
-	switch(sizeof(*(_p_))) {					\
+	switch (sizeof(*(_p_))) {					\
 	case 4:								\
 		_val_ = __arch_xchg(_p_, _val_);			\
 		break;							\
-- 
2.47.3

