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

Alexandre Ghiti
2025-01-30T14:18:59Z

Hi Andreas,

On 30/01/2025 10:25, Andreas Schwab 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. 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, \

That's a nice catch indeed. IIUC, we have the same issue here
https://elixir.bootlin.com/linux/v6.13/source/arch/riscv/include/asm/futex.h#L89
right?

hanks,

Alex