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

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Message-ID: <ea6baf8d-683b-45d8-9719-7dbf207c1ab5@linux.dev>
Date: Wed, 27 Aug 2025 01:33:21 +0800
From: Lance Yang <lance.yang@...ux.dev>
To: Eero Tamminen <oak@...sinkinet.fi>
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, Andrew Morton <akpm@...ux-foundation.org>,
 Finn Thain <fthain@...ux-m68k.org>
Subject: Re: [PATCH] atomic: Specify natural alignment for atomic_t

Hi Eero,

On 2025/8/26 23:22, Eero Tamminen wrote:
> 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@...l.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@...ux.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.

Thanks a lot for testing!

Cheers,
Lance