linux-kernel - [PATCH/RFC 6/7] arm64: Replace ACCESS_ONCE for spinlock code with barriers

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Message-Id: <1416834210-61738-7-git-send-email-borntraeger@de.ibm.com>
Date:	Mon, 24 Nov 2014 14:03:29 +0100
From:	Christian Borntraeger <borntraeger@...ibm.com>
To:	linux-kernel@...r.kernel.org
Cc:	"linux-arch@...r.kernel.org, linux-mips@...ux-mips.org, linux-x86_64@...r.kernel.org, linux-s390"@vger.kernel.org,
	Paolo Bonzini <pbonzini@...hat.com>,
	paulmck@...ux.vnet.ibm.com, mingo@...nel.org,
	torvalds@...ux-foundation.org,
	Catalin Marinas <catalin.marinas@....com>,
	Will Deacon <will.deacon@....com>,
	Christian Borntraeger <borntraeger@...ibm.com>
Subject: [PATCH/RFC 6/7] arm64: Replace ACCESS_ONCE for spinlock code with barriers

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 access the lock with a barrier.

Signed-off-by: Christian Borntraeger <borntraeger@...ibm.com>
---
 arch/arm64/include/asm/spinlock.h | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/arch/arm64/include/asm/spinlock.h b/arch/arm64/include/asm/spinlock.h
index c45b7b1..f72dc64 100644
--- a/arch/arm64/include/asm/spinlock.h
+++ b/arch/arm64/include/asm/spinlock.h
@@ -99,12 +99,15 @@ 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));
+	arch_spinlock_t lockval = *lock;
+	barrier();
+	return !arch_spin_value_unlocked(lockval);
 }
 
 static inline int arch_spin_is_contended(arch_spinlock_t *lock)
 {
-	arch_spinlock_t lockval = ACCESS_ONCE(*lock);
+	arch_spinlock_t lockval = *lock;
+	barrier();
 	return (lockval.next - lockval.owner) > 1;
 }
 #define arch_spin_is_contended	arch_spin_is_contended
-- 
1.9.3

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