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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>
wifi-calibration
Matthew Wilcox 2008-04-11 15:23:52 -04:00 committed by Matthew Wilcox
parent b17170b2fa
commit 714493cd54
2 changed files with 70 additions and 48 deletions
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39
include/linux/semaphore.h
View File

@ -4,8 +4,7 @@
*
* Distributed under the terms of the GNU GPL, version 2
*
* Counting semaphores allow up to <n> tasks to acquire the semaphore
* simultaneously.
* Please see kernel/semaphore.c for documentation of these functions
*/
#ifndef __LINUX_SEMAPHORE_H
#define __LINUX_SEMAPHORE_H
@ -13,11 +12,7 @@
#include <linux/list.h>
#include <linux/spinlock.h>
/*
* The spinlock controls access to the other members of the semaphore.
* 'count' represents how many more tasks can acquire this semaphore.
* Tasks waiting for the lock are kept on the wait_list.
*/
/* Please don't access any members of this structure directly */
struct semaphore {
spinlock_t lock;
unsigned int count;
@ -46,41 +41,11 @@ static inline void sema_init(struct semaphore *sem, int val)
#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_interruptible(), except the sleep may only be interrupted by
* signals which are fatal to this process.
*/
extern int __must_check down_killable(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);
/*
* As down(), except this function will return -ETIME if it fails to
* acquire the semaphore within the specified number of jiffies.
*/
extern int __must_check down_timeout(struct semaphore *sem, long jiffies);
/*
* 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 */

79
kernel/semaphore.c
View File

@ -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;