linux-kernel - Re: [PATCH 9/9] kernel: tighten rules for ACCESS ONCE

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Date:	Wed, 3 Dec 2014 16:16:21 -0800
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Christian Borntraeger <borntraeger@...ibm.com>
Cc:	linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org,
	torvalds@...ux-foundation.org
Subject: Re: [PATCH 9/9] kernel: tighten rules for ACCESS ONCE

On Wed, Dec 03, 2014 at 11:30:21PM +0100, Christian Borntraeger wrote:
> Now that all non-scalar users of ACCESS_ONCE have been converted
> to READ_ONCE or ASSIGN once, lets tighten ACCESS_ONCE to only
> work on scalar types.
> This variant was proposed by Alexei Starovoitov.
> 
> Signed-off-by: Christian Borntraeger <borntraeger@...ibm.com>

With or without the updated comment:

Reviewed-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>

> ---
>  include/linux/compiler.h | 10 +++++++++-
>  1 file changed, 9 insertions(+), 1 deletion(-)
> 
> diff --git a/include/linux/compiler.h b/include/linux/compiler.h
> index 947710e..5ba91de 100644
> --- a/include/linux/compiler.h
> +++ b/include/linux/compiler.h
> @@ -441,8 +441,16 @@ static __always_inline void __assign_once_size(volatile void *p, void *res, int
>   * merging, or refetching absolutely anything at any time.  Its main intended
>   * use is to mediate communication between process-level code and irq/NMI
>   * handlers, all running on the same CPU.

This comment is obsolete in the same way as that of READ_ONCE() and
ASSIGN_ONCE(), but probably more to the point to just get rid of
ACCESS_ONCE().  ;-)

> + *
> + * ACCESS_ONCE will only work on scalar types. For union types, ACCESS_ONCE
> + * on a union member will work as long as the size of the member matches the
> + * size of the union and the size is smaller than word size.
> + * If possible READ_ONCE/ASSIGN_ONCE should be used instead.
>   */
> -#define ACCESS_ONCE(x) (*(volatile typeof(x) *)&(x))
> +#define __ACCESS_ONCE(x) ({ \
> +	 __maybe_unused typeof(x) __var = 0; \
> +	(volatile typeof(x) *)&(x); })
> +#define ACCESS_ONCE(x) (*__ACCESS_ONCE(x))
> 
>  /* Ignore/forbid kprobes attach on very low level functions marked by this attribute: */
>  #ifdef CONFIG_KPROBES
> -- 
> 1.9.3
> 

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