linux-kernel - Re: [PATCH 8/9] s390/kvm: REPLACE ACCESS_ONCE with READ

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Date:	Wed, 3 Dec 2014 16:12:43 -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 8/9] s390/kvm: REPLACE ACCESS_ONCE with READ_ONCE

On Wed, Dec 03, 2014 at 11:30:20PM +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 ipte lock 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/s390/kvm/gaccess.c | 14 ++++++++------
>  1 file changed, 8 insertions(+), 6 deletions(-)
> 
> diff --git a/arch/s390/kvm/gaccess.c b/arch/s390/kvm/gaccess.c
> index 0f961a1..8f195fa 100644
> --- a/arch/s390/kvm/gaccess.c
> +++ b/arch/s390/kvm/gaccess.c
> @@ -229,10 +229,10 @@ static void ipte_lock_simple(struct kvm_vcpu *vcpu)
>  		goto out;
>  	ic = &vcpu->kvm->arch.sca->ipte_control;
>  	do {
> -		old = ACCESS_ONCE(*ic);
> +		old = READ_ONCE(*ic);
>  		while (old.k) {
>  			cond_resched();
> -			old = ACCESS_ONCE(*ic);
> +			old = READ_ONCE(*ic);
>  		}
>  		new = old;
>  		new.k = 1;
> @@ -251,7 +251,8 @@ static void ipte_unlock_simple(struct kvm_vcpu *vcpu)
>  		goto out;
>  	ic = &vcpu->kvm->arch.sca->ipte_control;
>  	do {
> -		new = old = ACCESS_ONCE(*ic);
> +		old = READ_ONCE(*ic);
> +		new = old;
>  		new.k = 0;
>  	} while (cmpxchg(&ic->val, old.val, new.val) != old.val);
>  	wake_up(&vcpu->kvm->arch.ipte_wq);
> @@ -265,10 +266,10 @@ static void ipte_lock_siif(struct kvm_vcpu *vcpu)
> 
>  	ic = &vcpu->kvm->arch.sca->ipte_control;
>  	do {
> -		old = ACCESS_ONCE(*ic);
> +		old = READ_ONCE(*ic);
>  		while (old.kg) {
>  			cond_resched();
> -			old = ACCESS_ONCE(*ic);
> +			old = READ_ONCE(*ic);
>  		}
>  		new = old;
>  		new.k = 1;
> @@ -282,7 +283,8 @@ static void ipte_unlock_siif(struct kvm_vcpu *vcpu)
> 
>  	ic = &vcpu->kvm->arch.sca->ipte_control;
>  	do {
> -		new = old = ACCESS_ONCE(*ic);
> +		old = READ_ONCE(*ic);
> +		new = old;
>  		new.kh--;
>  		if (!new.kh)
>  			new.k = 0;
> -- 
> 1.9.3
> 

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