From 8492c2845d805c540c115ca8c8efc552dbf2db0c Mon Sep 17 00:00:00 2001
From: Chris Robinson <chris.kcat@gmail.com>
Date: Tue, 22 Nov 2016 03:40:15 -0800
Subject: [PATCH] Avoid some unnecessary seq_cst memory ordering

---
 common/rwlock.c | 14 ++++++++------
 1 file changed, 8 insertions(+), 6 deletions(-)

diff --git a/common/rwlock.c b/common/rwlock.c
index cfa3aee4..f1a86fa6 100644
--- a/common/rwlock.c
+++ b/common/rwlock.c
@@ -11,10 +11,10 @@
 /* A simple spinlock. Yield the thread while the given integer is set by
  * another. Could probably be improved... */
 #define LOCK(l) do {                                                          \
-    while(ATOMIC_EXCHANGE(int, &(l), true) == true)                            \
+    while(ATOMIC_EXCHANGE(int, &(l), true, almemory_order_acq_rel) == true)   \
         althrd_yield();                                                       \
 } while(0)
-#define UNLOCK(l) ATOMIC_STORE(&(l), false)
+#define UNLOCK(l) ATOMIC_STORE(&(l), false, almemory_order_release)
 
 
 void RWLockInit(RWLock *lock)
@@ -30,7 +30,8 @@ void ReadLock(RWLock *lock)
 {
     LOCK(lock->read_entry_lock);
     LOCK(lock->read_lock);
-    if(IncrementRef(&lock->read_count) == 1)
+    /* NOTE: ATOMIC_ADD returns the *old* value! */
+    if(ATOMIC_ADD(&lock->read_count, 1, almemory_order_acq_rel) == 0)
         LOCK(lock->write_lock);
     UNLOCK(lock->read_lock);
     UNLOCK(lock->read_entry_lock);
@@ -38,13 +39,14 @@ void ReadLock(RWLock *lock)
 
 void ReadUnlock(RWLock *lock)
 {
-    if(DecrementRef(&lock->read_count) == 0)
+    /* NOTE: ATOMIC_SUB returns the *old* value! */
+    if(ATOMIC_SUB(&lock->read_count, 1, almemory_order_acq_rel) == 1)
         UNLOCK(lock->write_lock);
 }
 
 void WriteLock(RWLock *lock)
 {
-    if(IncrementRef(&lock->write_count) == 1)
+    if(ATOMIC_ADD(&lock->write_count, 1, almemory_order_acq_rel) == 0)
         LOCK(lock->read_lock);
     LOCK(lock->write_lock);
 }
@@ -52,6 +54,6 @@ void WriteLock(RWLock *lock)
 void WriteUnlock(RWLock *lock)
 {
     UNLOCK(lock->write_lock);
-    if(DecrementRef(&lock->write_count) == 0)
+    if(ATOMIC_SUB(&lock->write_count, 1, almemory_order_acq_rel) == 1)
         UNLOCK(lock->read_lock);
 }