x86/spinlock: 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 spinlock code to replace ACCESS_ONCE with READ_ONCE.

Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>
Acked-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

diff --git a/arch/x86/include/asm/spinlock.h b/arch/x86/include/asm/spinlock.h
index 9295016..12a69b4 100644 (file)
--- 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;
 }