Re: [PATCH] riscv/futex: sign extend compare value in atomic cmpxchg

Björn Töpel
2025-02-03T15:33:54Z

Andreas Schwab <schwab@suse.de> writes:

> Make sure the compare value in the lr/sc loop is sign extended to match
> what lr.w does. Fortunately, due to the compiler keeping the register
> contents sign extended anyway the lack of the explicit extension didn't
> result in wrong code so far, but this cannot be relied upon.
>
> Fixes: b90edb33010b ("RISC-V: Add futex support.")
> Signed-off-by: Andreas Schwab <schwab@suse.de>

Hmm, in this scenario we *can* rely on it, no (inline vs macro)?

Regardless, having an explicit cast there doesn't hurt, and make it more
obvious!

Reviewed-by: Björn Töpel <bjorn@rivosinc.com>

Let's add a link to Jessica's comment as well:
Link: https://lore.kernel.org/linux-riscv/CC2D9220-F8DE-4CC8-ACAD-7B1A21E276FE@jrtc27.com/

> ---
> arch/riscv/include/asm/futex.h | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/arch/riscv/include/asm/futex.h b/arch/riscv/include/asm/futex.h
> index fc8130f995c1..6907c456ac8c 100644
> --- a/arch/riscv/include/asm/futex.h
> +++ b/arch/riscv/include/asm/futex.h
> @@ -93,7 +93,7 @@ futex_atomic_cmpxchg_inatomic(u32 *uval, u32 __user *uaddr,
> _ASM_EXTABLE_UACCESS_ERR(1b, 3b, %[r]) \
> _ASM_EXTABLE_UACCESS_ERR(2b, 3b, %[r]) \
> : [r] "+r" (ret), [v] "=&r" (val), [u] "+m" (*uaddr), [t] "=&r" (tmp)
> - : [ov] "Jr" (oldval), [nv] "Jr" (newval)
> + : [ov] "Jr" ((long)(int)oldval), [nv] "Jr" (newval)
> : "memory");
> __disable_user_access();
>
> --
> 2.48.1
>
>
> --
> Andreas Schwab, SUSE Labs, schwab@suse.de
> GPG Key fingerprint = 0196 BAD8 1CE9 1970 F4BE 1748 E4D4 88E3 0EEA B9D7
> "And now for something completely different."
>
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