Improve semaphore documentation

Move documentation from semaphore.h to semaphore.c as requested by
Andrew Morton.  Also reformat to kernel-doc style and add some more
notes about the implementation.

Signed-off-by: Matthew Wilcox <willy@linux.intel.com>

kernel/semaphore.c

@@ -3,6 +3,26 @@
  * Author: Matthew Wilcox <willy@linux.intel.com>
  *
  * Distributed under the terms of the GNU GPL, version 2
+ *
+ * This file implements counting semaphores.
+ * A counting semaphore may be acquired 'n' times before sleeping.
+ * See mutex.c for single-acquisition sleeping locks which enforce
+ * rules which allow code to be debugged more easily.
+ */
+
+/*
+ * Some notes on the implementation:
+ *
+ * The spinlock controls access to the other members of the semaphore.
+ * down_trylock() and up() can be called from interrupt context, so we
+ * have to disable interrupts when taking the lock.  It turns out various
+ * parts of the kernel expect to be able to use down() on a semaphore in
+ * interrupt context when they know it will succeed, so we have to use
+ * irqsave variants for down(), down_interruptible() and down_killable()
+ * too.
+ *
+ * The ->count variable represents how many more tasks can acquire this
+ * semaphore.  If it's zero, there may be tasks waiting on the wait_list.
  */
 
 #include <linux/compiler.h>
@@ -12,22 +32,23 @@
 #include <linux/semaphore.h>
 #include <linux/spinlock.h>
 
-/*
- * Some notes on the implementation:
- *
- * down_trylock() and up() can be called from interrupt context.
- * So we have to disable interrupts when taking the lock.
- *
- * The ->count variable defines how many more tasks can acquire the
- * semaphore.  If it's zero, there may be tasks waiting on the list.
- */
-
 static noinline void __down(struct semaphore *sem);
 static noinline int __down_interruptible(struct semaphore *sem);
 static noinline int __down_killable(struct semaphore *sem);
 static noinline int __down_timeout(struct semaphore *sem, long jiffies);
 static noinline void __up(struct semaphore *sem);
 
+/**
+ * down - acquire the semaphore
+ * @sem: the semaphore to be acquired
+ *
+ * Acquires the semaphore.  If no more tasks are allowed to acquire the
+ * semaphore, calling this function will put the task to sleep until the
+ * semaphore is released.
+ *
+ * Use of this function is deprecated, please use down_interruptible() or
+ * down_killable() instead.
+ */
 void down(struct semaphore *sem)
 {
 	unsigned long flags;
@@ -41,6 +62,15 @@ void down(struct semaphore *sem)
 }
 EXPORT_SYMBOL(down);
 
+/**
+ * down_interruptible - acquire the semaphore unless interrupted
+ * @sem: the semaphore to be acquired
+ *
+ * Attempts to acquire the semaphore.  If no more tasks are allowed to
+ * acquire the semaphore, calling this function will put the task to sleep.
+ * If the sleep is interrupted by a signal, this function will return -EINTR.
+ * If the semaphore is successfully acquired, this function returns 0.
+ */
 int down_interruptible(struct semaphore *sem)
 {
 	unsigned long flags;
@@ -57,6 +87,16 @@ int down_interruptible(struct semaphore *sem)
 }
 EXPORT_SYMBOL(down_interruptible);
 
+/**
+ * down_killable - acquire the semaphore unless killed
+ * @sem: the semaphore to be acquired
+ *
+ * Attempts to acquire the semaphore.  If no more tasks are allowed to
+ * acquire the semaphore, calling this function will put the task to sleep.
+ * If the sleep is interrupted by a fatal signal, this function will return
+ * -EINTR.  If the semaphore is successfully acquired, this function returns
+ * 0.
+ */
 int down_killable(struct semaphore *sem)
 {
 	unsigned long flags;
@@ -78,7 +118,7 @@ EXPORT_SYMBOL(down_killable);
  * @sem: the semaphore to be acquired
  *
  * Try to acquire the semaphore atomically.  Returns 0 if the mutex has
- * been acquired successfully and 1 if it is contended.
+ * been acquired successfully or 1 if it it cannot be acquired.
  *
  * NOTE: This return value is inverted from both spin_trylock and
  * mutex_trylock!  Be careful about this when converting code.
@@ -101,6 +141,16 @@ int down_trylock(struct semaphore *sem)
 }
 EXPORT_SYMBOL(down_trylock);
 
+/**
+ * down_timeout - acquire the semaphore within a specified time
+ * @sem: the semaphore to be acquired
+ * @jiffies: how long to wait before failing
+ *
+ * Attempts to acquire the semaphore.  If no more tasks are allowed to
+ * acquire the semaphore, calling this function will put the task to sleep.
+ * If the semaphore is not released within the specified number of jiffies,
+ * this function returns -ETIME.  It returns 0 if the semaphore was acquired.
+ */
 int down_timeout(struct semaphore *sem, long jiffies)
 {
 	unsigned long flags;
@@ -117,6 +167,13 @@ int down_timeout(struct semaphore *sem, long jiffies)
 }
 EXPORT_SYMBOL(down_timeout);
 
+/**
+ * up - release the semaphore
+ * @sem: the semaphore to release
+ *
+ * Release the semaphore.  Unlike mutexes, up() may be called from any
+ * context and even by tasks which have never called down().
+ */
 void up(struct semaphore *sem)
 {
 	unsigned long flags;