linux-kernel - Re: [PATCH] locking/atomic: cmpxchg: Make __generic_cmpxchg_local compare against zero-extended 'old' value

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Message-Id: <a55d3d15-d89d-43c4-85e7-c6d18a57a32f@app.fastmail.com>
Date:   Fri, 03 Feb 2023 15:32:28 +0100
From:   "Arnd Bergmann" <arnd@...db.de>
To:     "Matt Evans" <mev@...osinc.com>, linux-kernel@...r.kernel.org,
        Linux-Arch <linux-arch@...r.kernel.org>
Cc:     "Palmer Dabbelt" <palmer@...osinc.com>
Subject: Re: [PATCH] locking/atomic: cmpxchg: Make __generic_cmpxchg_local compare
 against zero-extended 'old' value

On Wed, Feb 1, 2023, at 19:39, Matt Evans wrote:
> __generic_cmpxchg_local takes unsigned long old/new arguments which
> might end up being up-cast from smaller signed types (which will
> sign-extend).  The loaded compare value must be compared against a
> truncated smaller type, so down-cast appropriately for each size.
>
> The issue is apparent on 64-bit machines with code, such as
> atomic_dec_unless_positive(), that sign-extends from int.
>
> 64-bit machines generally don't use the generic cmpxchg but
> development/early ports might make use of it, so make it correct.
>
> Signed-off-by: Matt Evans <mev@...osinc.com>
> ---

Applied to the asm-generic tree for 6.3, thanks

    Arnd