[PATCH 4/4 V4] qspinlock for x86: smp_mb__after_spin_lock() is free

From	Manfred Spraul <manfred@colorfullife.com>
Newsgroups	linux.kernel
Subject	[PATCH 4/4 V4] qspinlock for x86: smp_mb__after_spin_lock() is free
Date	2016-08-29 15:40 +0200
Message-ID	<sbuX8-7SV-37@gated-at.bofh.it> (permalink)
References	<sbuX7-7SV-7@gated-at.bofh.it>
Organization	linux.* mail to news gateway

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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@colorfullife.com>
---
 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..04d26ed 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()	do { } while (0)
+#endif
+
 #include <asm-generic/qspinlock.h>
 
 #endif /* _ASM_X86_QSPINLOCK_H */
-- 
2.5.5

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[PATCH 0/4 V4] Clarify/standardize memory barriers for lock/unlock Manfred Spraul <manfred@colorfullife.com> - 2016-08-29 15:40 +0200
  [PATCH 2/4 V4] spinlock.h: Move smp_mb__after_unlock_lock to spinlock.h Manfred Spraul <manfred@colorfullife.com> - 2016-08-29 15:40 +0200
  [PATCH 1/4 v4] spinlock: Document memory barrier rules Manfred Spraul <manfred@colorfullife.com> - 2016-08-29 15:40 +0200
    Re: [PATCH 1/4 v4] spinlock: Document memory barrier rules Davidlohr Bueso <dave@stgolabs.net> - 2016-08-29 19:40 +0200
  [PATCH 4/4 V4] qspinlock for x86: smp_mb__after_spin_lock() is free Manfred Spraul <manfred@colorfullife.com> - 2016-08-29 15:40 +0200

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