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

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Message-Id: <A31D967F-B057-4C6D-B403-B4FB3973F185@jrtc27.com>
Date: Mon, 3 Feb 2025 21:25:00 +0000
From: Jessica Clarke <jrtc27@...c27.com>
To: Andreas Schwab <schwab@...e.de>
Cc: linux-riscv@...ts.infradead.org,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH] riscv/futex: sign extend compare value in atomic cmpxchg

On 3 Feb 2025, at 10:06, Andreas Schwab <schwab@...e.de> wrote:
> 
> 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.

GCC may guarantee this today, but LLVM does not, and definitely
generates code that does not do so. Whether that happens for any of the
consumers of this API today I don’t know, but it is definitely relying
on things that aren’t true for LLVM, and are not specified in the psABI.

Jess

> Fixes: b90edb33010b ("RISC-V: Add futex support.")
> Signed-off-by: Andreas Schwab <schwab@...e.de>
> ---
> 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@...e.de
> GPG Key fingerprint = 0196 BAD8 1CE9 1970 F4BE  1748 E4D4 88E3 0EEA B9D7
> "And now for something completely different."
> 
> _______________________________________________
> linux-riscv mailing list
> linux-riscv@...ts.infradead.org
> http://lists.infradead.org/mailman/listinfo/linux-riscv

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