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

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Message-Id: <20250827114516.efd544acda4e3c0492d893e7@kernel.org>
Date: Wed, 27 Aug 2025 11:45:16 +0900
From: Masami Hiramatsu (Google) <mhiramat@...nel.org>
To: Finn Thain <fthain@...ux-m68k.org>
Cc: "Andrew Morton" <akpm@...ux-foundation.org>, "Lance Yang"
 <lance.yang@...ux.dev>, "Geert Uytterhoeven" <geert@...ux-m68k.org>,
 "Masami Hiramatsu" <mhiramat@...nel.org>, "Eero Tamminen"
 <oak@...sinkinet.fi>, "Peter Zijlstra" <peterz@...radead.org>,
 "Will Deacon" <will@...nel.org>, stable@...r.kernel.org,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH] atomic: Specify natural alignment for atomic_t

On Mon, 25 Aug 2025 12:03:05 +1000
Finn Thain <fthain@...ux-m68k.org> 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.
> 
> Cc: Andrew Morton <akpm@...ux-foundation.org>
> Cc: Geert Uytterhoeven <geert@...ux-m68k.org>
> Cc: Lance Yang <lance.yang@...ux.dev>
> Cc: Masami Hiramatsu <mhiramat@...nel.org>
> Cc: Eero Tamminen <oak@...sinkinet.fi>
> Cc: Peter Zijlstra <peterz@...radead.org>
> Cc: Will Deacon <will@...nel.org>
> Cc: stable@...r.kernel.org
> 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>

This seems good anyway because unaligned atomic memory access
sounds insane. So looks good to me.

Reviewed-by: Masami Hiramatsu (Google) <mhiramat@...nel.org>

Anyway, Lance's patch[1] is still needed. We'd better gracefully
ignore if the blocker is not aligned, because hung_task blocker
detection is an optional for debugging and not necessary for
the kernel operation.

[1] https://lore.kernel.org/all/20250823050036.7748-1-lance.yang@linux.dev/ 


Thank you,

> ---
> 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.
> ---
>  include/linux/types.h | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/include/linux/types.h b/include/linux/types.h
> index 6dfdb8e8e4c3..cd5b2b0f4b02 100644
> --- a/include/linux/types.h
> +++ b/include/linux/types.h
> @@ -179,7 +179,7 @@ typedef phys_addr_t resource_size_t;
>  typedef unsigned long irq_hw_number_t;
>  
>  typedef struct {
> -	int counter;
> +	int counter __aligned(sizeof(int));
>  } atomic_t;
>  
>  #define ATOMIC_INIT(i) { (i) }
> -- 
> 2.49.1
> 


-- 
Masami Hiramatsu (Google) <mhiramat@...nel.org>