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

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Date:	Wed,  3 Dec 2014 23:30:20 +0100
From:	Christian Borntraeger <borntraeger@...ibm.com>
To:	linux-kernel@...r.kernel.org
Cc:	linux-arch@...r.kernel.org, paulmck@...ux.vnet.ibm.com,
	torvalds@...ux-foundation.org,
	Christian Borntraeger <borntraeger@...ibm.com>
Subject: [PATCH 8/9] s390/kvm: REPLACE ACCESS_ONCE with READ_ONCE

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>
---
 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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