linux-kernel - RE: [PATCH] asm-generic: avoid __generic_cmpxchg

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Message-ID: <60996fd3604b403ab67e8f73854264ee@AcuMS.aculab.com>
Date:   Thu, 2 Mar 2023 16:40:00 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Arnd Bergmann' <arnd@...nel.org>, Will Deacon <will@...nel.org>,
        "Peter Zijlstra" <peterz@...radead.org>,
        Arnd Bergmann <arnd@...db.de>, Matt Evans <mev@...osinc.com>
CC:     Boqun Feng <boqun.feng@...il.com>,
        Mark Rutland <mark.rutland@....com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>
Subject: RE: [PATCH] asm-generic: avoid __generic_cmpxchg_local warnings

...
> I had another look at why the cast is even needed for atomic_cmpxchg(),
> and as Matt describes the problem here is that atomic_t contains a
> signed 'int', but cmpxchg() takes an 'unsigned long' argument, and
> converting between the two leads to a 64-bit sign-extension of
> negative 32-bit atomics.

How about:
	signed_var + 0u + 0ull;

Converts 32bit signed to 64bit unsigned without sign extension.
Compiler will throw it all away if not needed.

	David

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