x86: fix spin_is_contended()

commit 7bc069c6 upstream

The masked difference is what needs to be compared against 1, rather
than the difference of masked values (which can be negative).
Signed-off-by: Jan Beulich <jbeulich@novell.com>
Acked-by: Nick Piggin <npiggin@suse.de>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

include/asm-x86/spinlock.h

@@ -65,7 +65,7 @@ static inline int __raw_spin_is_contended(raw_spinlock_t *lock)
 {
 	int tmp = ACCESS_ONCE(lock->slock);
 
-	return (((tmp >> 8) & 0xff) - (tmp & 0xff)) > 1;
+	return (((tmp >> 8) - tmp) & 0xff) > 1;
 }
 
 static __always_inline void __raw_spin_lock(raw_spinlock_t *lock)
@@ -129,7 +129,7 @@ static inline int __raw_spin_is_contended(raw_spinlock_t *lock)
 {
 	int tmp = ACCESS_ONCE(lock->slock);
 
-	return (((tmp >> 16) & 0xffff) - (tmp & 0xffff)) > 1;
+	return (((tmp >> 16) - tmp) & 0xffff) > 1;
 }
 
 static __always_inline void __raw_spin_lock(raw_spinlock_t *lock)