[PATCH] asm-generic: avoid __generic_cmpxchg_local warnings

[PATCH] asm-generic: avoid __generic_cmpxchg_local warnings

[Arnd Bergmann]

From: Arnd Bergmann <arnd@arndb.de>

Code that passes a 32-bit constant into cmpxchg() produces a harmless
sparse warning because of the truncation in the branch that is not taken:

fs/erofs/zdata.c: note: in included file (through /home/arnd/arm-soc/arch/arm/include/asm/cmpxchg.h, /home/arnd/arm-soc/arch/arm/include/asm/atomic.h, /home/arnd/arm-soc/include/linux/atomic.h, ...):
include/asm-generic/cmpxchg-local.h:29:33: warning: cast truncates bits from constant value (5f0ecafe becomes fe)
include/asm-generic/cmpxchg-local.h:33:34: warning: cast truncates bits from constant value (5f0ecafe becomes cafe)
include/asm-generic/cmpxchg-local.h:29:33: warning: cast truncates bits from constant value (5f0ecafe becomes fe)
include/asm-generic/cmpxchg-local.h:30:42: warning: cast truncates bits from constant value (5f0edead becomes ad)
include/asm-generic/cmpxchg-local.h:33:34: warning: cast truncates bits from constant value (5f0ecafe becomes cafe)
include/asm-generic/cmpxchg-local.h:34:44: warning: cast truncates bits from constant value (5f0edead becomes dead)

This was reported as a regression to Matt's recent __generic_cmpxchg_local
patch, though this patch only added more warnings on top of the ones
that were already there.

Rewording the truncation to use an explicit bitmask instead of a cast
to a smaller type avoids the warning but otherwise leaves the code
unchanged.

I had another look at why the cast is even needed for atomic_cmpxchg(),
and as Matt describes the problem here is that atomic_t contains a
signed 'int', but cmpxchg() takes an 'unsigned long' argument, and
converting between the two leads to a 64-bit sign-extension of
negative 32-bit atomics.

I checked the other implementations of arch_cmpxchg() and did not find
any others that run into the same problem as __generic_cmpxchg_local(),
but it's easy to be on the safe side here and always convert the
signed int into an unsigned int when calling arch_cmpxchg(), as this
will work even when any of the arch_cmpxchg() implementations run
into the same problem.

Fixes: 624654152284 ("locking/atomic: cmpxchg: Make __generic_cmpxchg_local compare against zero-extended 'old' value")
Reviewed-by: Matt Evans <mev@rivosinc.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
---
 include/asm-generic/atomic.h        |  4 ++--
 include/asm-generic/cmpxchg-local.h | 12 ++++++------
 include/asm-generic/cmpxchg.h       |  6 +++---
 3 files changed, 11 insertions(+), 11 deletions(-)

diff --git a/include/asm-generic/atomic.h b/include/asm-generic/atomic.h
index 04b8be9f1a77..e271d6708c87 100644
--- a/include/asm-generic/atomic.h
+++ b/include/asm-generic/atomic.h
@@ -130,7 +130,7 @@ ATOMIC_OP(xor, ^)
 #define arch_atomic_read(v)			READ_ONCE((v)->counter)
 #define arch_atomic_set(v, i)			WRITE_ONCE(((v)->counter), (i))
 
-#define arch_atomic_xchg(ptr, v)		(arch_xchg(&(ptr)->counter, (v)))
-#define arch_atomic_cmpxchg(v, old, new)	(arch_cmpxchg(&((v)->counter), (old), (new)))
+#define arch_atomic_xchg(ptr, v)		(arch_xchg(&(ptr)->counter, (u32)(v)))
+#define arch_atomic_cmpxchg(v, old, new)	(arch_cmpxchg(&((v)->counter), (u32)(old), (u32)(new)))
 
 #endif /* __ASM_GENERIC_ATOMIC_H */
diff --git a/include/asm-generic/cmpxchg-local.h b/include/asm-generic/cmpxchg-local.h
index c3e7315b7c1d..3df9f59a544e 100644
--- a/include/asm-generic/cmpxchg-local.h
+++ b/include/asm-generic/cmpxchg-local.h
@@ -26,16 +26,16 @@ static inline unsigned long __generic_cmpxchg_local(volatile void *ptr,
 	raw_local_irq_save(flags);
 	switch (size) {
 	case 1: prev = *(u8 *)ptr;
-		if (prev == (u8)old)
-			*(u8 *)ptr = (u8)new;
+		if (prev == (old & 0xffu))
+			*(u8 *)ptr = (new & 0xffu);
 		break;
 	case 2: prev = *(u16 *)ptr;
-		if (prev == (u16)old)
-			*(u16 *)ptr = (u16)new;
+		if (prev == (old & 0xffffu))
+			*(u16 *)ptr = (new & 0xffffu);
 		break;
 	case 4: prev = *(u32 *)ptr;
-		if (prev == (u32)old)
-			*(u32 *)ptr = (u32)new;
+		if (prev == (old & 0xffffffffffu))
+			*(u32 *)ptr = (new & 0xffffffffu);
 		break;
 	case 8: prev = *(u64 *)ptr;
 		if (prev == old)
diff --git a/include/asm-generic/cmpxchg.h b/include/asm-generic/cmpxchg.h
index dca4419922a9..848de25fc4bf 100644
--- a/include/asm-generic/cmpxchg.h
+++ b/include/asm-generic/cmpxchg.h
@@ -32,7 +32,7 @@ unsigned long __generic_xchg(unsigned long x, volatile void *ptr, int size)
 #else
 		local_irq_save(flags);
 		ret = *(volatile u8 *)ptr;
-		*(volatile u8 *)ptr = x;
+		*(volatile u8 *)ptr = (x & 0xffu);
 		local_irq_restore(flags);
 		return ret;
 #endif /* __xchg_u8 */
@@ -43,7 +43,7 @@ unsigned long __generic_xchg(unsigned long x, volatile void *ptr, int size)
 #else
 		local_irq_save(flags);
 		ret = *(volatile u16 *)ptr;
-		*(volatile u16 *)ptr = x;
+		*(volatile u16 *)ptr = (x & 0xffffu);
 		local_irq_restore(flags);
 		return ret;
 #endif /* __xchg_u16 */
@@ -54,7 +54,7 @@ unsigned long __generic_xchg(unsigned long x, volatile void *ptr, int size)
 #else
 		local_irq_save(flags);
 		ret = *(volatile u32 *)ptr;
-		*(volatile u32 *)ptr = x;
+		*(volatile u32 *)ptr = (x & 0xffffffffu);
 		local_irq_restore(flags);
 		return ret;
 #endif /* __xchg_u32 */
-- 
2.39.2

RE: [PATCH] asm-generic: avoid __generic_cmpxchg_local warnings

[David Laight]

...
> I had another look at why the cast is even needed for atomic_cmpxchg(),
> and as Matt describes the problem here is that atomic_t contains a
> signed 'int', but cmpxchg() takes an 'unsigned long' argument, and
> converting between the two leads to a 64-bit sign-extension of
> negative 32-bit atomics.

How about:
	signed_var + 0u + 0ull;

Converts 32bit signed to 64bit unsigned without sign extension.
Compiler will throw it all away if not needed.

	David

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