Generic semaphore implementation

Semaphores are no longer performance-critical, so a generic C
implementation is better for maintainability, debuggability and
extensibility.  Thanks to Peter Zijlstra for fixing the lockdep
warning.  Thanks to Harvey Harrison for pointing out that the
unlikely() was unnecessary.

Signed-off-by: Matthew Wilcox <willy@linux.intel.com>
Acked-by: Ingo Molnar <mingo@elte.hu>

include/linux/semaphore.h

@@ -0,0 +1,77 @@
+/*
+ * Copyright (c) 2008 Intel Corporation
+ * Author: Matthew Wilcox <willy@linux.intel.com>
+ *
+ * Distributed under the terms of the GNU GPL, version 2
+ *
+ * Counting semaphores allow up to <n> tasks to acquire the semaphore
+ * simultaneously.
+ */
+#ifndef __LINUX_SEMAPHORE_H
+#define __LINUX_SEMAPHORE_H
+
+#include <linux/list.h>
+#include <linux/spinlock.h>
+
+/*
+ * The spinlock controls access to the other members of the semaphore.
+ * 'count' is decremented by every task which calls down*() and incremented
+ * by every call to up().  Thus, if it is positive, it indicates how many
+ * more tasks may acquire the lock.  If it is negative, it indicates how
+ * many tasks are waiting for the lock.  Tasks waiting for the lock are
+ * kept on the wait_list.
+ */
+struct semaphore {
+	spinlock_t		lock;
+	int			count;
+	struct list_head	wait_list;
+};
+
+#define __SEMAPHORE_INITIALIZER(name, n)				\
+{									\
+	.lock		= __SPIN_LOCK_UNLOCKED((name).lock),		\
+	.count		= n,						\
+	.wait_list	= LIST_HEAD_INIT((name).wait_list),		\
+}
+
+#define __DECLARE_SEMAPHORE_GENERIC(name, count) \
+	struct semaphore name = __SEMAPHORE_INITIALIZER(name, count)
+
+#define DECLARE_MUTEX(name)	__DECLARE_SEMAPHORE_GENERIC(name, 1)
+
+static inline void sema_init(struct semaphore *sem, int val)
+{
+	static struct lock_class_key __key;
+	*sem = (struct semaphore) __SEMAPHORE_INITIALIZER(*sem, val);
+	lockdep_init_map(&sem->lock.dep_map, "semaphore->lock", &__key, 0);
+}
+
+#define init_MUTEX(sem)		sema_init(sem, 1)
+#define init_MUTEX_LOCKED(sem)	sema_init(sem, 0)
+
+/*
+ * Attempt to acquire the semaphore.  If another task is already holding the
+ * semaphore, sleep until the semaphore is released.
+ */
+extern void down(struct semaphore *sem);
+
+/*
+ * As down(), except the sleep may be interrupted by a signal.  If it is,
+ * this function will return -EINTR.
+ */
+extern int __must_check down_interruptible(struct semaphore *sem);
+
+/*
+ * As down(), except this function will not sleep.  It will return 0 if it
+ * acquired the semaphore and 1 if the semaphore was contended.  This
+ * function may be called from any context, including interrupt and softirq.
+ */
+extern int __must_check down_trylock(struct semaphore *sem);
+
+/*
+ * Release the semaphore.  Unlike mutexes, up() may be called from any
+ * context and even by tasks which have never called down().
+ */
+extern void up(struct semaphore *sem);
+
+#endif /* __LINUX_SEMAPHORE_H */