Re: [PATCH 3/9] x86/spinlock: Replace ACCESS_ONCE with READ_ONCE

[Paul E. McKenney]

On Wed, Dec 03, 2014 at 11:30:15PM +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@xxxxxxxxxx>

Acked-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>

> ---
>  arch/x86/include/asm/spinlock.h | 8 ++++----
>  1 file changed, 4 insertions(+), 4 deletions(-)
> 
> diff --git a/arch/x86/include/asm/spinlock.h b/arch/x86/include/asm/spinlock.h
> index 9295016..12a69b4 100644
> --- a/arch/x86/include/asm/spinlock.h
> +++ b/arch/x86/include/asm/spinlock.h
> @@ -92,7 +92,7 @@ static __always_inline void arch_spin_lock(arch_spinlock_t *lock)
>  		unsigned count = SPIN_THRESHOLD;
> 
>  		do {
> -			if (ACCESS_ONCE(lock->tickets.head) == inc.tail)
> +			if (READ_ONCE(lock->tickets.head) == inc.tail)
>  				goto out;
>  			cpu_relax();
>  		} while (--count);
> @@ -105,7 +105,7 @@ static __always_inline int arch_spin_trylock(arch_spinlock_t *lock)
>  {
>  	arch_spinlock_t old, new;
> 
> -	old.tickets = ACCESS_ONCE(lock->tickets);
> +	old.tickets = READ_ONCE(lock->tickets);
>  	if (old.tickets.head != (old.tickets.tail & ~TICKET_SLOWPATH_FLAG))
>  		return 0;
> 
> @@ -162,14 +162,14 @@ static __always_inline void arch_spin_unlock(arch_spinlock_t *lock)
> 
>  static inline int arch_spin_is_locked(arch_spinlock_t *lock)
>  {
> -	struct __raw_tickets tmp = ACCESS_ONCE(lock->tickets);
> +	struct __raw_tickets tmp = READ_ONCE(lock->tickets);
> 
>  	return tmp.tail != tmp.head;
>  }
> 
>  static inline int arch_spin_is_contended(arch_spinlock_t *lock)
>  {
> -	struct __raw_tickets tmp = ACCESS_ONCE(lock->tickets);
> +	struct __raw_tickets tmp = READ_ONCE(lock->tickets);
> 
>  	return (__ticket_t)(tmp.tail - tmp.head) > TICKET_LOCK_INC;
>  }
> -- 
> 1.9.3
> 

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