[ltt-dev] [RFC PATCH 3/7] urcu:implement userspace lock

[Lai Jiangshan]

The pthread APIs don't have proxies APIs, so I have to
reinvent the wheel.

Signed-off-by: Lai Jiangshan <laijs at cn.fujitsu.com>
---
 urcu.c             |   82 ++++++++++++++++++++++++++++++++++++++++++++++++++++
 urcu/futex.h       |    2 +
 urcu/static/urcu.h |    5 +++
 3 files changed, 89 insertions(+), 0 deletions(-)

diff --git a/urcu.c b/urcu.c
index 064a155..bd54467 100644
--- a/urcu.c
+++ b/urcu.c
@@ -337,6 +337,88 @@ out:
 }
 #else /* #ifndef URCU_WAIT_READER */
 
+static inline void urcu_sync_proxy_lock(int32_t *lock, int32_t proxy_owner)
+{
+	assert(_CMM_LOAD_SHARED(*lock) == 0);
+	_CMM_STORE_SHARED(*lock, proxy_owner);
+}
+
+static inline void urcu_sync_proxy_unlock(int32_t *lock)
+{
+	_CMM_STORE_SHARED(*lock, 0);
+}
+
+static void urcu_sync_lock(int32_t *lock, int32_t self)
+{
+	for (;;) {
+		int32_t o = _CMM_LOAD_SHARED(*lock);
+
+		if (likely(o == 0)) {
+			o = uatomic_cmpxchg(lock, 0, self);
+			if (likely(o == 0))
+				return;
+		}
+
+		if ((o & FUTEX_WAITERS) == 0) {
+			if (uatomic_cmpxchg(lock, o, o | FUTEX_WAITERS) != o)
+				continue;
+		}
+
+		futex_async(lock, FUTEX_WAIT, o | FUTEX_WAITERS, NULL, NULL, 0);
+	}
+}
+
+/*
+ * returns:
+ * -2:  error, lock is held by other.
+ * -1:  error, lock is not held by anyone.
+ *  0:  success unlock.
+ *  1:  failed to unlock when have waiters.
+ */
+static int urcu_sync_try_unlock(int32_t *lock, int32_t self)
+{
+	int32_t o = _CMM_LOAD_SHARED(*lock);
+
+	if (likely(o == self)) {
+		o = uatomic_cmpxchg(lock, self, 0);
+		if (likely(o == self))
+			return 0;
+	}
+
+	if (o == (self | FUTEX_WAITERS))
+		return 1;
+	if (!o)
+		return -1;
+	else
+		return -2;
+}
+
+static void urcu_sync_slow_unlock(int32_t *lock)
+{
+	_CMM_STORE_SHARED(*lock, 0);
+	futex_async(lock, FUTEX_WAKE, 1, NULL, NULL, 0);
+}
+
+static void urcu_sync_unlock(int32_t *lock, int32_t self)
+{
+	int ret = urcu_sync_try_unlock(lock, self);
+
+	assert(ret == 0 || ret == 1);
+
+	if (unlikely(ret == 1))
+		urcu_sync_slow_unlock(lock);
+}
+
+static void urcu_sync_unlock_if_not_proxy_unlocked(int32_t *lock, int32_t self)
+{
+	int ret = urcu_sync_try_unlock(lock, self);
+
+	assert(ret == -1 || ret == 0 || ret == 1);
+
+	if (unlikely(ret == 1))
+		urcu_sync_slow_unlock(lock);
+}
+
 #endif /* #else #ifndef URCU_WAIT_READER */
 
 /*
diff --git a/urcu/futex.h b/urcu/futex.h
index 98acc12..e47c307 100644
--- a/urcu/futex.h
+++ b/urcu/futex.h
@@ -31,6 +31,8 @@ extern "C" {
 #define FUTEX_WAIT		0
 #define FUTEX_WAKE		1
 
+#define FUTEX_WAITERS		0x80000000
+
 /*
  * sys_futex compatibility header.
  * Use *only* *either of* futex_noasync OR futex_async on a given address.
diff --git a/urcu/static/urcu.h b/urcu/static/urcu.h
index ebdefbe..32d1af8 100644
--- a/urcu/static/urcu.h
+++ b/urcu/static/urcu.h
@@ -308,6 +308,11 @@ static inline void _rcu_read_unlock(void)
 }
 #else /* #ifndef URCU_WAIT_READER */
 
+static inline int32_t urcu_sync_lock_onwer(int32_t *lock)
+{
+	return _CMM_LOAD_SHARED(*lock) & ~FUTEX_WAITERS;
+}
+
 #endif /* #else #ifndef URCU_WAIT_READER */
 
 #ifdef __cplusplus
-- 
1.7.4.4