[PATCH v2 3/5] csky: Emulate two-byte cmpxchg

[Bradley Morgan]

On 17 September 2026 10:04:35 BST, David Laight
<david.laight.linux at gmail.com> wrote:
>On Wed, 16 Sep 2026 20:02:13 +0000
>Bradley Morgan <brads at mainlining.org> wrote:
>
>> 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 now takes natural u16
>> arguments, so the (uintptr_t) casts on __old and __new become (u16)
>> casts in all three switch instances.
>> 
>> Signed-off-by: Bradley Morgan <brads at 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..e53628268e8c 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, (u16)__old, (u16)__new); \
>> +		break;						\
>
>Did you check that compiles?
>It needs to compile without a warning when exchanging a pointer type.
>I think it would be better to make both __old and __new 'unsigned long'
>adding a cast in the definition/assignment to convert pointers.
>aka:
>	unsigned long __old = (unsigned long)(old);
>
>(The cast on the call is separate from the type of the function
>parameters.)
>
>Not a new bug, but there also doesn't seem to be a pointer-integer type
>check here (there is probably one in another architecture).
>
>I think that means these compile:
>	int i;
>	int *p;
>	...
>	cmpxchg(&p, 4, 5);
>	cmpxchg(&i, p, 5);
>
>Simplest fix is probably:
>	unsigned long __old = (unsigned long)(0 ? *__ptr : (old));
>
>David
>

Makes sense. I'll just do a V3 adding tags and your fix, cause csky ? And
the main patch is the one having issues in sashiko.



>
>>  	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, (u16)__old, (u16)__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, (u16)__old, (u16)__new); \
>> +		break;						\
>>  	case 4:							\
>>  		asm volatile (					\
>>  		RELEASE_FENCE					\
>
>

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