linux-kernel - Re: [PATCH 7/9] arm/spinlock: Replace ACCESS_ONCE with READ

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Date:	Wed, 3 Dec 2014 16:12:02 -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 7/9] arm/spinlock: Replace ACCESS_ONCE with READ_ONCE

On Wed, Dec 03, 2014 at 11:30:19PM +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 spinlock 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/arm/include/asm/spinlock.h | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/arm/include/asm/spinlock.h b/arch/arm/include/asm/spinlock.h
> index ac4bfae..0fa4184 100644
> --- a/arch/arm/include/asm/spinlock.h
> +++ b/arch/arm/include/asm/spinlock.h
> @@ -120,12 +120,12 @@ static inline int arch_spin_value_unlocked(arch_spinlock_t lock)
> 
>  static inline int arch_spin_is_locked(arch_spinlock_t *lock)
>  {
> -	return !arch_spin_value_unlocked(ACCESS_ONCE(*lock));
> +	return !arch_spin_value_unlocked(READ_ONCE(*lock));
>  }
> 
>  static inline int arch_spin_is_contended(arch_spinlock_t *lock)
>  {
> -	struct __raw_tickets tickets = ACCESS_ONCE(lock->tickets);
> +	struct __raw_tickets tickets = READ_ONCE(lock->tickets);
>  	return (tickets.next - tickets.owner) > 1;
>  }
>  #define arch_spin_is_contended	arch_spin_is_contended
> -- 
> 1.9.3
> 

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