x86/spinlock: Replace ACCESS_ONCE with READ_ONCE

borntraeger · borntraeger · commit 4f9d1382e6f8 · 2014-12-18T09:54:38.000+01:00
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
@@ -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;
 }

Original file line number	Diff line number	Diff line change
`@@ -92,7 +92,7 @@ static __always_inline void arch_spin_lock(arch_spinlock_t *lock)`
`92`	`92`	`unsigned count = SPIN_THRESHOLD;`
`93`	`93`
`94`	`94`	`do {`
`95`		`- if (ACCESS_ONCE(lock->tickets.head) == inc.tail)`
	`95`	`+ if (READ_ONCE(lock->tickets.head) == inc.tail)`
`96`	`96`	`goto out;`
`97`	`97`	`cpu_relax();`
`98`	`98`	`} while (--count);`
`@@ -105,7 +105,7 @@ static __always_inline int arch_spin_trylock(arch_spinlock_t *lock)`
`105`	`105`	`{`
`106`	`106`	`arch_spinlock_t old, new;`
`107`	`107`
`108`		`- old.tickets = ACCESS_ONCE(lock->tickets);`
	`108`	`+ old.tickets = READ_ONCE(lock->tickets);`
`109`	`109`	`if (old.tickets.head != (old.tickets.tail & ~TICKET_SLOWPATH_FLAG))`
`110`	`110`	`return 0;`
`111`	`111`
`@@ -162,14 +162,14 @@ static __always_inline void arch_spin_unlock(arch_spinlock_t *lock)`
`162`	`162`
`163`	`163`	`static inline int arch_spin_is_locked(arch_spinlock_t *lock)`
`164`	`164`	`{`
`165`		`- struct __raw_tickets tmp = ACCESS_ONCE(lock->tickets);`
	`165`	`+ struct __raw_tickets tmp = READ_ONCE(lock->tickets);`
`166`	`166`
`167`	`167`	`return tmp.tail != tmp.head;`
`168`	`168`	`}`
`169`	`169`
`170`	`170`	`static inline int arch_spin_is_contended(arch_spinlock_t *lock)`
`171`	`171`	`{`
`172`		`- struct __raw_tickets tmp = ACCESS_ONCE(lock->tickets);`
	`172`	`+ struct __raw_tickets tmp = READ_ONCE(lock->tickets);`
`173`	`173`
`174`	`174`	`return (__ticket_t)(tmp.tail - tmp.head) > TICKET_LOCK_INC;`
`175`	`175`	`}`