[PATCHv2 04/10] x86/spinlock: Replace ACCESS_ONCE with READ_ONCE/ASSIGN_ONCE

From: Christian Borntraeger
Date: Tue Nov 25 2014 - 07:42:22 EST


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
and ASSIGN_ONCE.

Signed-off-by: Christian Borntraeger <borntraeger@xxxxxxxxxx>
---
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..af6e673 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 (ASSIGN_ONCE(inc.tail, lock->tickets.head))
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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