From bdccbf7356dda7b89036be770b62d6305e786318 Mon Sep 17 00:00:00 2001
From: Balazs Gerofi <bgerofi@riken.jp>
Date: Wed, 10 Apr 2019 20:26:13 +0900
Subject: [PATCH] MCS: fix ARM64 issue by using smp_XXX() functions (i.e.,
 barrier()s)

Change-Id: I41470c082308c7c1ac91f88db2229958398d2e68
---
 arch/x86_64/kernel/include/arch/cpu.h |  8 +++++
 lib/include/ihk/lock.h                | 42 ++++++++++++++++-----------
 2 files changed, 33 insertions(+), 17 deletions(-)

diff --git a/arch/x86_64/kernel/include/arch/cpu.h b/arch/x86_64/kernel/include/arch/cpu.h
index 1114c29b..1debb1fd 100644
--- a/arch/x86_64/kernel/include/arch/cpu.h
+++ b/arch/x86_64/kernel/include/arch/cpu.h
@@ -42,4 +42,12 @@ static inline unsigned long read_tsc(void)
 	___p1;								\
 })
 
+#define smp_store_release(p, v)			\
+({							\
+	compiletime_assert_atomic_type(*p);	\
+	barrier();							\
+	WRITE_ONCE(*p, v);					\
+})
+
+
 #endif /* ARCH_CPU_H */
diff --git a/lib/include/ihk/lock.h b/lib/include/ihk/lock.h
index 4a25e085..5c93d842 100644
--- a/lib/include/ihk/lock.h
+++ b/lib/include/ihk/lock.h
@@ -46,6 +46,7 @@ static void mcs_lock_init(struct mcs_lock_node *node)
 #endif // SPIN_LOCK_IN_MCS
 }
 
+
 static void __mcs_lock_lock(struct mcs_lock_node *lock,
 		struct mcs_lock_node *node)
 {
@@ -56,15 +57,20 @@ static void __mcs_lock_lock(struct mcs_lock_node *lock,
 
 	node->next = NULL;
 	node->locked = 0;
-	__atomic_exchange(&(lock->next), &node, &pred, __ATOMIC_SEQ_CST);
 
-	if (pred) {
-		node->locked = 1;
-		pred->next = node;
-		while (node->locked != 0) {
-			cpu_pause();
-		}
+	pred = xchg(&lock->next, node);
+	if (likely(pred == NULL)) {
+		/*
+		 * Lock acquired, don't need to set node->locked to 1. Threads
+		 * only spin on its own node->locked value for lock acquisition.
+		 */
+		return;
 	}
+	WRITE_ONCE(pred->next, node);
+
+	/* Wait until the lock holder passes the lock down. */
+	while (!(smp_load_acquire(&node->locked)))
+		cpu_pause();
 #endif // SPIN_LOCK_IN_MCS
 }
 
@@ -74,20 +80,22 @@ static void __mcs_lock_unlock(struct mcs_lock_node *lock,
 #ifdef SPIN_LOCK_IN_MCS
 	ihk_mc_spinlock_unlock_noirq(&lock->spinlock);
 #else
-	if (node->next == NULL) {
-		struct mcs_lock_node *desired = NULL;
-		struct mcs_lock_node *expected = node;
-		if (__atomic_compare_exchange(&(lock->next), &expected, &desired, 0,
-					__ATOMIC_SEQ_CST, __ATOMIC_SEQ_CST)) {
-			return;
-		}
+	struct mcs_lock_node *next = READ_ONCE(node->next);
 
-		while (node->next == NULL) {
+	if (likely(!next)) {
+		/*
+		 * Release the lock by setting it to NULL
+		 */
+		if (likely(cmpxchg(&lock->next, node, NULL) == node))
+			return;
+
+		/* Wait until the next pointer is set */
+		while (!(next = READ_ONCE(node->next)))
 			cpu_pause();
-		}
 	}
 
-	node->next->locked = 0;
+	/* Pass lock to next waiter. */
+	smp_store_release((&next->locked), 1);
 #endif // SPIN_LOCK_IN_MCS
 }