Re: [PATCH] riscv/atomic: Do proper sign extension also for unsigned in arch_cmpxchg

[David Laight <david.laight.linux@gmail.com>]

On Thu, 30 Jan 2025 10:25:38 +0100
Andreas Schwab <schwab@suse.de> wrote:

> Sign extend also an unsigned compare value to match what lr.w is doing.
> Otherwise try_cmpxchg may spuriously return true when used on a u32 value
> that has the sign bit set, as it happens often in inode_set_ctime_current.
> 
> Do this in three conversion steps.  The first conversion to long is needed
> to avoid a -Wpointer-to-int-cast warning when arch_cmpxchg is used with a
> pointer type.

Doesn't that break things by discarding the high bits of a pointer value?

	David

> Then convert to int and back to long to always sign extend
> the 32-bit value to 64-bit.
> 
> Fixes: 6c58f25e6938 ("riscv/atomic: Fix sign extension for RV64I")
> Signed-off-by: Andreas Schwab <schwab@suse.de>
> ---
>  arch/riscv/include/asm/cmpxchg.h | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/arch/riscv/include/asm/cmpxchg.h b/arch/riscv/include/asm/cmpxchg.h
> index 4cadc56220fe..427c41dde643 100644
> --- a/arch/riscv/include/asm/cmpxchg.h
> +++ b/arch/riscv/include/asm/cmpxchg.h
> @@ -231,7 +231,7 @@
>  		__arch_cmpxchg(".w", ".w" sc_sfx, ".w" cas_sfx,		\
>  			       sc_prepend, sc_append,			\
>  			       cas_prepend, cas_append,			\
> -			       __ret, __ptr, (long), __old, __new);	\
> +			       __ret, __ptr, (long)(int)(long), __old, __new);	\
>  		break;							\
>  	case 8:								\
>  		__arch_cmpxchg(".d", ".d" sc_sfx, ".d" cas_sfx,		\


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