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Mon, 22 Sep 2025 11:22:05 -0400 (EDT) X-Mailer: MessagingEngine.com Webmail Interface Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 X-ThreadId: Ah_uXtpTlO07 Date: Mon, 22 Sep 2025 17:21:45 +0200 From: "Arnd Bergmann" To: "Finn Thain" , "Geert Uytterhoeven" Cc: "Peter Zijlstra" , "Will Deacon" , "Andrew Morton" , "Boqun Feng" , "Jonathan Corbet" , "Mark Rutland" , linux-kernel@vger.kernel.org, Linux-Arch , linux-m68k@vger.kernel.org, "Lance Yang" Message-Id: <57ac401b-222b-4c85-b719-9e671a4617cf@app.fastmail.com> In-Reply-To: <6c295a4e-4d18-a004-5db8-db2e57afc957@linux-m68k.org> References: <6c295a4e-4d18-a004-5db8-db2e57afc957@linux-m68k.org> Subject: Re: [RFC v2 2/3] atomic: Specify alignment for atomic_t and atomic64_t Content-Type: text/plain Content-Transfer-Encoding: 7bit On Mon, Sep 22, 2025, at 10:16, Finn Thain wrote: > > Yes, I noticed that also, after I started building with defconfigs. > I pushed a new patch to my github repo. > > https://github.com/fthain/linux/tree/atomic_t I had a look at that repository, and I think the extra annotations you added on a lot of structures have the same mistake that I made when looking at annotating ABI structures for -malign-int earlier: Adding __aligned__((sizeof(long))) to a structure definition only changes the /outer/ alignment of the structure itself, so e.g. struct { short a; int b; } __attribute__((aligned(sizeof(long)))); creates a structure with 4-byte alignment and 8-byte size when building with -mno-align-int, but the padding is added after 'b', which is still misaligned. In order to align members the same way that -malign-int does, each unaligned member needs a separate attribute to force aligning it, like struct { short a; int b __attribute__((aligned(sizeof(int)))); }; This is different from how __attribute__((packed)) works, as that is always applied to each member of the struct if you add it to the end. Arnd