--- v2.6.18-rc2-mm1~ori/include/linux/lw_rwlock.h	1970-01-01 01:00:00.000000000 +0100
+++ v2.6.18-rc2-mm1/include/linux/lw_rwlock.h	2006-07-28 17:25:04.000000000 +0200
@@ -0,0 +1,71 @@
+#ifndef __LINUX_LW_RWLOCK_H
+#define __LINUX_LW_RWLOCK_H
+
+/*
+ * LightWeight readwriter lock.
+ * The strategy is: modifications while the lock is held are short, do not
+ * sleep and veeery rare, but read access should be free of any exclusive
+ * locks.
+ * The original implementation was written for net/socket.c
+ */
+
+#include <linux/spinlock.h>
+
+
+struct lw_rwlock {
+	/* tracks down the number of current readers */
+	atomic_t readers;
+	/* the actual lock, only held by writers */
+	spinlock_t lock;
+};
+
+#define __LW_RWLOCK_UNLOCKED(lockname) \
+		 { { 0 }, __SPIN_LOCK_UNLOCKED(lockname) }
+
+#define lw_rwlock_init(x) \
+		do { *(x) = (lw_rwlock_t) __LW_RWLOCK_UNLOCKED(x); } while (0)
+
+#define DEFINE_LW_RWLOCK(x) \
+		struct lw_rwlock x = __LW_RWLOCK_UNLOCKED(x)
+
+#if defined(CONFIG_SMP) || defined(CONFIG_PREEMPT)
+
+static inline void lw_write_lock(struct lw_rwlock *l)
+{
+	spin_lock(&l->lock);
+	while (atomic_read(&l->readers) != 0) {
+		spin_unlock(&l->lock);
+
+		yield();
+
+		spin_lock(&l->lock);
+	}
+}	
+
+static inline void lw_write_unlock(struct lw_rwlock *l) 
+{
+	spin_unlock(&l->lock);
+}
+
+static inline void lw_read_lock(struct lw_rwlock *l)
+{
+	atomic_inc(&l->readers);
+	spin_unlock_wait(&l->lock);
+}
+
+static inline void lw_read_unlock(struct lw_rwlock *l)
+{
+	atomic_dec(&l->readers);
+}
+
+#else
+
+#define lw_write_lock(x) do { } while(0)
+#define lw_write_unlock(x) do { } while(0)
+#define lw_read_lock(x) do { } while(0)
+#define lw_read_unlock() do { } while(0)
+
+#endif /* CONFIG_SMP || CONFIG_PREEMPT */
+
+
+#endif /* __LINUX_LW_RWLOCK_H */