linux-kernel - [PATCH 3.10 060/319] arm64: spinlocks: implement smp_mb__before_spinlock() as smp

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Date:   Sun,  5 Feb 2017 20:20:47 +0100
From:   Willy Tarreau <w@....eu>
To:     linux-kernel@...r.kernel.org, stable@...r.kernel.org,
        linux@...ck-us.net
Cc:     Will Deacon <will.deacon@....com>,
        Peter Zijlstra <peterz@...radead.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Willy Tarreau <w@....eu>
Subject: [PATCH 3.10 060/319] arm64: spinlocks: implement smp_mb__before_spinlock() as smp_mb()

From: Will Deacon <will.deacon@....com>

commit 872c63fbf9e153146b07f0cece4da0d70b283eeb upstream.

smp_mb__before_spinlock() is intended to upgrade a spin_lock() operation
to a full barrier, such that prior stores are ordered with respect to
loads and stores occuring inside the critical section.

Unfortunately, the core code defines the barrier as smp_wmb(), which
is insufficient to provide the required ordering guarantees when used in
conjunction with our load-acquire-based spinlock implementation.

This patch overrides the arm64 definition of smp_mb__before_spinlock()
to map to a full smp_mb().

Cc: Peter Zijlstra <peterz@...radead.org>
Reported-by: Alan Stern <stern@...land.harvard.edu>
Signed-off-by: Will Deacon <will.deacon@....com>
Signed-off-by: Catalin Marinas <catalin.marinas@....com>
Signed-off-by: Willy Tarreau <w@....eu>
---
 arch/arm64/include/asm/spinlock.h | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/arch/arm64/include/asm/spinlock.h b/arch/arm64/include/asm/spinlock.h
index 0defa07..c3cab6f 100644
--- a/arch/arm64/include/asm/spinlock.h
+++ b/arch/arm64/include/asm/spinlock.h
@@ -200,4 +200,14 @@ static inline int arch_read_trylock(arch_rwlock_t *rw)
 #define arch_read_relax(lock)	cpu_relax()
 #define arch_write_relax(lock)	cpu_relax()
 
+/*
+ * Accesses appearing in program order before a spin_lock() operation
+ * can be reordered with accesses inside the critical section, by virtue
+ * of arch_spin_lock being constructed using acquire semantics.
+ *
+ * In cases where this is problematic (e.g. try_to_wake_up), an
+ * smp_mb__before_spinlock() can restore the required ordering.
+ */
+#define smp_mb__before_spinlock()	smp_mb()
+
 #endif /* __ASM_SPINLOCK_H */
-- 
2.8.0.rc2.1.gbe9624a