linux-kernel - Re: [PATCH] atomic: Specify natural alignment for atomic

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Message-ID: <1a5ce56a-d0d0-481e-b663-a7b176682a65@helsinkinet.fi>
Date: Tue, 26 Aug 2025 18:22:45 +0300
From: Eero Tamminen <oak@...sinkinet.fi>
To: Finn Thain <fthain@...ux-m68k.org>,
 Andrew Morton <akpm@...ux-foundation.org>, Lance Yang <lance.yang@...ux.dev>
Cc: Geert Uytterhoeven <geert@...ux-m68k.org>,
 Masami Hiramatsu <mhiramat@...nel.org>, Peter Zijlstra
 <peterz@...radead.org>, Will Deacon <will@...nel.org>,
 stable@...r.kernel.org, linux-kernel@...r.kernel.org,
 linux-m68k@...ts.linux-m68k.org
Subject: Re: [PATCH] atomic: Specify natural alignment for atomic_t

Hi Finn & Lance,

On 25.8.2025 5.03, Finn Thain wrote:
> Some recent commits incorrectly assumed the natural alignment of locks.
> That assumption fails on Linux/m68k (and, interestingly, would have failed
> on Linux/cris also). This leads to spurious warnings from the hang check
> code. Fix this bug by adding the necessary 'aligned' attribute.
[...]
> Reported-by: Eero Tamminen <oak@...sinkinet.fi>
> Closes: https://lore.kernel.org/lkml/CAMuHMdW7Ab13DdGs2acMQcix5ObJK0O2dG_Fxzr8_g58Rc1_0g@mail.gmail.com/
> Fixes: e711faaafbe5 ("hung_task: replace blocker_mutex with encoded blocker")
> Signed-off-by: Finn Thain <fthain@...ux-m68k.org>
> ---
> I tested this on m68k using GCC and it fixed the problem for me. AFAIK,
> the other architectures naturally align ints already so I'm expecting to
> see no effect there.

Yes, it fixes both of the issues (warnings & broken console):
	Tested-by: Eero Tamminen <oak@...sinkinet.fi>

(Emulated Atari Falcon) boot up performance with this is within normal 
variation.


On 23.8.2025 10.49, Lance Yang wrote:
 > Anyway, I've prepared two patches for discussion, either of which should
 > fix the alignment issue :)
 >
 > Patch A[1] adjusts the runtime checks to handle unaligned pointers.
 > Patch B[2] enforces 4-byte alignment on the core lock structures.
 >
 > Both tested on x86-64.
 >
 > [1] 
https://lore.kernel.org/lkml/20250823050036.7748-1-lance.yang@linux.dev
 > [2] https://lore.kernel.org/lkml/20250823074048.92498-1-
 > lance.yang@...ux.dev

Same goes for both of these, except that removing warnings makes minimal 
kernel boot 1-2% faster than 4-aligning the whole struct.


	- Eero