[PATCH v6 5/5] MCS Lock: Allows for architecture specific mcs lockand unlock

[Tim Chen]

Restructure code to allow for architecture specific defines
of the arch_mcs_spin_lock and arch_mcs_spin_unlock funtion
that can be optimized for specific architecture.  These
arch specific functions can be placed in asm/mcs_spinlock.h.
Otherwise the default arch_mcs_spin_lock and arch_mcs_spin_unlock
will be used.

Signed-off-by: Tim Chen <tim.c.chen@xxxxxxxxxxxxxxx>
---
 arch/Kconfig                  |  3 ++
 include/linux/mcs_spinlock.h  |  5 +++
 kernel/locking/mcs_spinlock.c | 93 +++++++++++++++++++++++++------------------
 3 files changed, 62 insertions(+), 39 deletions(-)

diff --git a/arch/Kconfig b/arch/Kconfig
index ded747c..c96c696 100644
--- a/arch/Kconfig
+++ b/arch/Kconfig
@@ -306,6 +306,9 @@ config HAVE_CMPXCHG_LOCAL
 config HAVE_CMPXCHG_DOUBLE
 	bool
 
+config HAVE_ARCH_MCS_LOCK
+	bool
+
 config ARCH_WANT_IPC_PARSE_VERSION
 	bool
 
diff --git a/include/linux/mcs_spinlock.h b/include/linux/mcs_spinlock.h
index d54bb23..d64786a 100644
--- a/include/linux/mcs_spinlock.h
+++ b/include/linux/mcs_spinlock.h
@@ -12,6 +12,11 @@
 #ifndef __LINUX_MCS_SPINLOCK_H
 #define __LINUX_MCS_SPINLOCK_H
 
+/* arch specific mcs lock and unlock functions defined here */
+#ifdef CONFIG_HAVE_ARCH_MCS_LOCK
+#include <asm/mcs_spinlock.h>
+#endif
+
 struct mcs_spinlock {
 	struct mcs_spinlock *next;
 	int locked; /* 1 if lock acquired */
diff --git a/kernel/locking/mcs_spinlock.c b/kernel/locking/mcs_spinlock.c
index 6f2ce8e..582584a 100644
--- a/kernel/locking/mcs_spinlock.c
+++ b/kernel/locking/mcs_spinlock.c
@@ -29,28 +29,36 @@
  * on this node->locked until the previous lock holder sets the node->locked
  * in mcs_spin_unlock().
  */
+#ifndef arch_mcs_spin_lock
+#define arch_mcs_spin_lock(lock, node)					\
+{									\
+	struct mcs_spinlock *prev;					\
+									\
+	/* Init node */							\
+	node->locked = 0;						\
+	node->next   = NULL;						\
+									\
+	/* xchg() provides a memory barrier */				\
+	prev = xchg(lock, node);					\
+	if (likely(prev == NULL)) {					\
+		/* Lock acquired */					\
+		return;							\
+	}								\
+	ACCESS_ONCE(prev->next) = node;					\
+	/*								\
+	 * Wait until the lock holder passes the lock down.		\
+	 * Using smp_load_acquire() provides a memory barrier that	\
+	 * ensures subsequent operations happen after the lock is	\
+	 * acquired.							\
+	 */								\
+	while (!(smp_load_acquire(&node->locked)))			\
+		arch_mutex_cpu_relax();					\
+}
+#endif
+
 void mcs_spin_lock(struct mcs_spinlock **lock, struct mcs_spinlock *node)
 {
-	struct mcs_spinlock *prev;
-
-	/* Init node */
-	node->locked = 0;
-	node->next   = NULL;
-
-	/* xchg() provides a memory barrier */
-	prev = xchg(lock, node);
-	if (likely(prev == NULL)) {
-		/* Lock acquired */
-		return;
-	}
-	ACCESS_ONCE(prev->next) = node;
-	/*
-	 * Wait until the lock holder passes the lock down.
-	 * Using smp_load_acquire() provides a memory barrier that
-	 * ensures subsequent operations happen after the lock is acquired.
-	 */
-	while (!(smp_load_acquire(&node->locked)))
-		arch_mutex_cpu_relax();
+	arch_mcs_spin_lock(lock, node);
 }
 EXPORT_SYMBOL_GPL(mcs_spin_lock);
 
@@ -58,26 +66,33 @@ EXPORT_SYMBOL_GPL(mcs_spin_lock);
  * Releases the lock. The caller should pass in the corresponding node that
  * was used to acquire the lock.
  */
+#ifndef arch_mcs_spin_unlock
+#define arch_mcs_spin_unlock(lock, node)				\
+{									\
+	struct mcs_spinlock *next = ACCESS_ONCE(node->next);		\
+									\
+	if (likely(!next)) {						\
+		/*							\
+		 * Release the lock by setting it to NULL               \
+		 */							\
+		if (likely(cmpxchg(lock, node, NULL) == node))          \
+			return;                                         \
+		/* Wait until the next pointer is set */		\
+		while (!(next = ACCESS_ONCE(node->next)))		\
+			arch_mutex_cpu_relax();				\
+	}								\
+	/*								\
+	 * Pass lock to next waiter.					\
+	 * smp_store_release() provides a memory barrier to ensure	\
+	 * all operations in the critical section has been completed	\
+	 * before unlocking.						\
+	 */								\
+	smp_store_release(&next->locked, 1);				\
+}
+#endif
+
 void mcs_spin_unlock(struct mcs_spinlock **lock, struct mcs_spinlock *node)
 {
-	struct mcs_spinlock *next = ACCESS_ONCE(node->next);
-
-	if (likely(!next)) {
-		/*
-		 * Release the lock by setting it to NULL
-		 */
-		if (likely(cmpxchg(lock, node, NULL) == node))
-			return;
-		/* Wait until the next pointer is set */
-		while (!(next = ACCESS_ONCE(node->next)))
-			arch_mutex_cpu_relax();
-	}
-	/*
-	 * Pass lock to next waiter.
-	 * smp_store_release() provides a memory barrier to ensure
-	 * all operations in the critical section has been completed
-	 * before unlocking.
-	 */
-	smp_store_release(&next->locked, 1);
+	arch_mcs_spin_unlock(lock, node);
 }
 EXPORT_SYMBOL_GPL(mcs_spin_unlock);
-- 
1.7.11.7


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