Re: [PATCH 2/3] asm-generic, x86: wrap atomic operations

[Mark Rutland <mark.rutland@arm.com>]

Hi,

On Tue, Mar 14, 2017 at 08:24:13PM +0100, Dmitry Vyukov wrote:
>  /**
> - * atomic_read - read atomic variable
> + * arch_atomic_read - read atomic variable
>   * @v: pointer of type atomic_t
>   *
>   * Atomically reads the value of @v.
>   */
> -static __always_inline int atomic_read(const atomic_t *v)
> +static __always_inline int arch_atomic_read(const atomic_t *v)
>  {
> -	return READ_ONCE((v)->counter);
> +	/*
> +	 * We use READ_ONCE_NOCHECK() because atomic_read() contains KASAN
> +	 * instrumentation. Double instrumentation is unnecessary.
> +	 */
> +	return READ_ONCE_NOCHECK((v)->counter);
>  }

Just to check, we do this to avoid duplicate reports, right?

If so, double instrumentation isn't solely "unnecessary"; it has a
functional difference, and we should explicitly describe that in the
comment.

... or are duplicate reports supressed somehow?

[...]

> +static __always_inline void arch_atomic_set(atomic_t *v, int i)
>  {
> +	/*
> +	 * We could use WRITE_ONCE_NOCHECK() if it exists, similar to
> +	 * READ_ONCE_NOCHECK() in arch_atomic_read(). But there is no such
> +	 * thing at the moment, and introducing it for this case does not
> +	 * worth it.
> +	 */
>  	WRITE_ONCE(v->counter, i);
>  }

If we are trying to avoid duplicate reports, we should do the same here.

[...]

> +static __always_inline short int atomic_inc_short(short int *v)
> +{
> +	return arch_atomic_inc_short(v);
> +}

This is x86-specific, and AFAICT, not used anywhere.

Given that it is arch-specific, I don't think it should be instrumented
here. If it isn't used, we could get rid of it entirely...

Thanks,
Mark.

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