linux-kernel - Re: [PATCH 3/6] locking/atomic/x86: Use READ_ONCE before atomic{,64}_try

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Message-ID: <ZhUhWcjo556WoTF4@FVFF77S0Q05N>
Date: Tue, 9 Apr 2024 12:07:05 +0100
From: Mark Rutland <mark.rutland@....com>
To: Uros Bizjak <ubizjak@...il.com>
Cc: x86@...nel.org, linux-kernel@...r.kernel.org,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...nel.org>, Borislav Petkov <bp@...en8.de>,
	Dave Hansen <dave.hansen@...ux.intel.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH 3/6] locking/atomic/x86: Use READ_ONCE before
 atomic{,64}_try_cmpxchg loops

On Tue, Apr 09, 2024 at 12:03:54PM +0200, Uros Bizjak wrote:
> The value preload before the cmpxchg loop does not need to be atomic,
> but should use READ_ONCE to prevent compiler from merging, refetching
> or reordering the read.
> 

Yes, and that's what arch_atomic_read() and arch_atomic64_read() do...

> This patch unifies arch_atomic{,64}_{,fetch}_{and,or,xor}() macros
> between x86_32 and x86_64 targets.
> 
> No functional changes intended.
> 
> Signed-off-by: Uros Bizjak <ubizjak@...il.com>
> Cc: Thomas Gleixner <tglx@...utronix.de>
> Cc: Ingo Molnar <mingo@...nel.org>
> Cc: Borislav Petkov <bp@...en8.de>
> Cc: Dave Hansen <dave.hansen@...ux.intel.com>
> Cc: "H. Peter Anvin" <hpa@...or.com>
> Cc: Peter Zijlstra <peterz@...radead.org>
> ---
>  arch/x86/include/asm/atomic.h      |  8 ++++----
>  arch/x86/include/asm/atomic64_64.h | 20 ++++++++++----------
>  2 files changed, 14 insertions(+), 14 deletions(-)
> 
> diff --git a/arch/x86/include/asm/atomic.h b/arch/x86/include/asm/atomic.h
> index 55a55ec04350..b166da21ee98 100644
> --- a/arch/x86/include/asm/atomic.h
> +++ b/arch/x86/include/asm/atomic.h
> @@ -20,7 +20,7 @@ static __always_inline int arch_atomic_read(const atomic_t *v)
>  	 * Note for KASAN: we deliberately don't use READ_ONCE_NOCHECK() here,
>  	 * it's non-inlined function that increases binary size and stack usage.
>  	 */
> -	return __READ_ONCE((v)->counter);
> +	return __READ_ONCE(v->counter);

Removing the unncessary brackets is fine, but the commit message didn't mention this.

[...]

>  static __always_inline int arch_atomic_fetch_and(int i, atomic_t *v)
>  {
> -	int val = arch_atomic_read(v);
> +	int val = __READ_ONCE(v->counter);

This is the wrong thing to do; arch_atomic_read() already has the required
semantic, and it more clearly aligns with the use of arch_atomic_try_cmpxchg()
below. It contains the documentation regarding why we use __READ_ONCE()
specifically (which we should probably note in arch_atomic64_read()).

Please leave this as-is, and likewise for the other cases below. Similarly, the
prior patch should use arch_atomic{,_64}_read() rather than using
__READ_ONCE().

[...]

>  static __always_inline s64 arch_atomic64_fetch_and(s64 i, atomic64_t *v)
>  {
> -	s64 val = arch_atomic64_read(v);
> +	s64 val = __READ_ONCE(v->counter);
> +
> +	do { } while (!arch_atomic64_try_cmpxchg(v, &val, val & i));
>  
> -	do {
> -	} while (!arch_atomic64_try_cmpxchg(v, &val, val & i));
>  	return val;

I think this reformatting is what you meant in the commit message when you said:

| This patch unifies arch_atomic{,64}_{,fetch}_{and,or,xor}() macros
| between x86_32 and x86_64 targets.

Assuming so, can you please jsut do that, and say:

  This patch reformats the x86_64 arch_atomic{,64}_{,fetch}_{and,or,xor}()
  functions to match the x86_32 versions.

Mark.