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

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Message-ID: <mvmbjvj2ft7.fsf@suse.de>
Date: Mon, 03 Feb 2025 16:44:52 +0100
From: Andreas Schwab <schwab@...e.de>
To: Björn Töpel <bjorn@...nel.org>
Cc: linux-riscv@...ts.infradead.org,  linux-kernel@...r.kernel.org
Subject: Re: [PATCH] riscv/futex: sign extend compare value in atomic cmpxchg

On Feb 03 2025, Björn Töpel wrote:

> Andreas Schwab <schwab@...e.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@...e.de>
>
> Hmm, in this scenario we *can* rely on it, no (inline vs macro)?

No, the issue is that the asm operand (oldval) is u32, but the asm is
using the 64-bit value from the register.  You cannot expect that the
compiler keeps the upper half defined in any way at this point.  That is
different for the operand that is passed in from newval, because sc.w is
only using the low 32-bits from the operand.

-- 
Andreas Schwab, SUSE Labs, schwab@...e.de
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