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

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Message-ID: <20250901094032.GL4068168@noisy.programming.kicks-ass.net>
Date: Mon, 1 Sep 2025 11:40:32 +0200
From: Peter Zijlstra <peterz@...radead.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>, 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, Sep 01, 2025 at 11:36:00AM +0200, Peter Zijlstra wrote:

> Something like the completely untested below should do I suppose.
> 
> ---
> diff --git a/include/linux/instrumented.h b/include/linux/instrumented.h
> index 711a1f0d1a73..e39cdfe5a59e 100644
> --- a/include/linux/instrumented.h
> +++ b/include/linux/instrumented.h
> @@ -67,6 +67,7 @@ static __always_inline void instrument_atomic_read(const volatile void *v, size_
>  {
>  	kasan_check_read(v, size);
>  	kcsan_check_atomic_read(v, size);
> +	WARN_ON_ONCE(IS_ENABLED(CONFIG_DEBUG_ATOMIC) && ((unsigned long)v & 3));
>  }
>  
>  /**
> @@ -81,6 +82,7 @@ static __always_inline void instrument_atomic_write(const volatile void *v, size
>  {
>  	kasan_check_write(v, size);
>  	kcsan_check_atomic_write(v, size);
> +	WARN_ON_ONCE(IS_ENABLED(CONFIG_DEBUG_ATOMIC) && ((unsigned long)v & 3));
>  }
>  
>  /**
> @@ -95,6 +97,7 @@ static __always_inline void instrument_atomic_read_write(const volatile void *v,
>  {
>  	kasan_check_write(v, size);
>  	kcsan_check_atomic_read_write(v, size);
> +	WARN_ON_ONCE(IS_ENABLED(CONFIG_DEBUG_ATOMIC) && ((unsigned long)v & 3));
>  }
>  
>  /**

Arguably, that should've been something like:

	((unsigned long)v & (size-1))

I suppose.

> diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
> index dc0e0c6ed075..1c7e30cdfe04 100644
> --- a/lib/Kconfig.debug
> +++ b/lib/Kconfig.debug
> @@ -1363,6 +1363,16 @@ config DEBUG_PREEMPT
>  	  depending on workload as it triggers debugging routines for each
>  	  this_cpu operation. It should only be used for debugging purposes.
>  
> +config DEBUG_ATOMIC
> +	bool "Debug atomic variables"
> +	depends on DEBUG_KERNEL
> +	help
> +	  If you say Y here then the kernel will add a runtime alignment check
> +	  to atomic accesses. Useful for architectures that do not have trap on
> +	  mis-aligned access.
> +
> +	  This option has potentially significant overhead.
> +
>  menu "Lock Debugging (spinlocks, mutexes, etc...)"
>  
>  config LOCK_DEBUGGING_SUPPORT