[PATCH 4/4] qspinlock for x86: smp_mb__after_spin_lock() is free

[Manfred Spraul]

For x86 qspinlocks, no additional memory barrier is required in
smp_mb__after_spin_lock:

Theoretically, for qspinlock we could define two barriers:
- smp_mb__after_spin_lock: Free for x86, not free for powerpc
- smp_mb__between_spin_lock_and_spin_unlock_wait():
	Free for all archs, see queued_spin_unlock_wait for details.

As smp_mb__between_spin_lock_and_spin_unlock_wait() is not used
in any hotpaths, the patch does not create that define yet.

Signed-off-by: Manfred Spraul <manfred@xxxxxxxxxxxxxxxx>
---
 arch/x86/include/asm/qspinlock.h | 11 +++++++++++
 1 file changed, 11 insertions(+)

diff --git a/arch/x86/include/asm/qspinlock.h b/arch/x86/include/asm/qspinlock.h
index eaba080..da06433 100644
--- a/arch/x86/include/asm/qspinlock.h
+++ b/arch/x86/include/asm/qspinlock.h
@@ -61,6 +61,17 @@ static inline bool virt_spin_lock(struct qspinlock *lock)
 }
 #endif /* CONFIG_PARAVIRT */
 
+#ifndef smp_mb__after_spin_lock
+/**
+ * smp_mb__after_spin_lock() - Provide smp_mb() after spin_lock
+ *
+ * queued_spin_lock() provides full memory barriers semantics,
+ * thus no further memory barrier is required. See
+ * queued_spin_unlock_wait() for further details.
+ */
+#define smp_mb__after_spin_lock()	barrier()
+#endif
+
 #include <asm-generic/qspinlock.h>
 
 #endif /* _ASM_X86_QSPINLOCK_H */
-- 
2.5.5