linux-kernel - Re: [PATCH 4/9] x86/gup: Replace ACCESS_ONCE with READ

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Date:	Wed, 3 Dec 2014 16:10:51 -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 4/9] x86/gup: Replace ACCESS_ONCE with READ_ONCE

On Wed, Dec 03, 2014 at 11:30:16PM +0100, Christian Borntraeger wrote:
> ACCESS_ONCE does not work reliably on non-scalar types. For
> example gcc 4.6 and 4.7 might remove the volatile tag for such
> accesses during the SRA (scalar replacement of aggregates) step
> (https://gcc.gnu.org/bugzilla/show_bug.cgi?id=58145)
> 
> Change the gup code to replace ACCESS_ONCE with READ_ONCE.
> 
> Signed-off-by: Christian Borntraeger <borntraeger@...ibm.com>

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

> ---
>  arch/x86/mm/gup.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/arch/x86/mm/gup.c b/arch/x86/mm/gup.c
> index 207d9aef..d754782 100644
> --- a/arch/x86/mm/gup.c
> +++ b/arch/x86/mm/gup.c
> @@ -15,7 +15,7 @@
>  static inline pte_t gup_get_pte(pte_t *ptep)
>  {
>  #ifndef CONFIG_X86_PAE
> -	return ACCESS_ONCE(*ptep);
> +	return READ_ONCE(*ptep);
>  #else
>  	/*
>  	 * With get_user_pages_fast, we walk down the pagetables without taking
> -- 
> 1.9.3
> 

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