asm-generic: avoid __generic_cmpxchg_local warnings

From: Arnd Bergmann <[email protected]>

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.

---
 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(-)

index 04b8be9f1a77..e271d6708c87 100644
@@ -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 */

index c3e7315b7c1d..3df9f59a544e 100644
@@ -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)

index dca4419922a9..848de25fc4bf 100644
@@ -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

...

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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