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authorMatthew Wilcox <matthew@wil.cx>2008-04-11 15:23:52 -0400
committerMatthew Wilcox <willy@linux.intel.com>2008-04-17 10:43:01 -0400
commit714493cd5468f42ca3c4f730a9c17c203abd5059 (patch)
tree9d087ad3b74cb1bd6fbd9be486fc2b361c6407dd /kernel/semaphore.c
parentb17170b2fac96705db3188f093f89e8e838418e4 (diff)
downloadlinux-2.6.34-ux500-714493cd5468f42ca3c4f730a9c17c203abd5059.tar.gz
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>
Diffstat (limited to 'kernel/semaphore.c')
-rw-r--r--kernel/semaphore.c79
1 files changed, 68 insertions, 11 deletions
diff --git a/kernel/semaphore.c b/kernel/semaphore.c
index bef977b1696..5c2942e768c 100644
--- a/kernel/semaphore.c
+++ b/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;