- include/linux/semaphore.h
/*
* Copyright (c) 2008 Intel Corporation
* Author: Matthew Wilcox <willy@linux.intel.com>
*
* Distributed under the terms of the GNU GPL, version 2
*
* Please see kernel/semaphore.c for documentation of these functions
*/
#ifndef __LINUX_SEMAPHORE_H
#define __LINUX_SEMAPHORE_H
#include <linux/list.h>
#include <linux/spinlock.h>
/* Please don't access any members of this structure directly */
struct semaphore {
raw_spinlock_t lock; /*操作信号量计数使需用自旋锁保护*/
unsigned int count; /*信号量计数*/
struct list_head wait_list; /*等待任务列表*/
};
#define __SEMAPHORE_INITIALIZER(name, n) \
{ \
.lock = __RAW_SPIN_LOCK_UNLOCKED((name).lock), \
.count = n, \
.wait_list = LIST_HEAD_INIT((name).wait_list), \
}
#define DEFINE_SEMAPHORE(name) \
struct semaphore name = __SEMAPHORE_INITIALIZER(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);
}
extern void down(struct semaphore *sem);
extern int __must_check down_interruptible(struct semaphore *sem);
extern int __must_check down_killable(struct semaphore *sem);
extern int __must_check down_trylock(struct semaphore *sem);
extern int __must_check down_timeout(struct semaphore *sem, long jiffies);
extern void up(struct semaphore *sem);
#endif /* __LINUX_SEMAPHORE_H */- include/linux/spinlock_types.h
...
#define __RAW_SPIN_LOCK_INITIALIZER(lockname) \
{ \
.raw_lock = __ARCH_SPIN_LOCK_UNLOCKED, \
SPIN_DEBUG_INIT(lockname) \
SPIN_DEP_MAP_INIT(lockname) }
...- arch/x86/include/asm/spinlock_types.h
...
#define __ARCH_SPIN_LOCK_UNLOCKED { { 0 } }
...-
成员
.lock是自旋锁，用来控制对信号量其他成员的访问。 -
成员
.count是信号量计数，表示有多少任务正持有锁。 -
如果
.count等于0，意味着有许多任务在等待列表上。 -
down()一族的函数会引起睡眠，因此不能在中断上下文中被调用，但是down_trylock()除外，因为其无论得不得的到锁都不会引起睡眠，因此可以在中断上下文中使用。 -
不像互斥量，
up()可能在任何上下文中被调用，甚至是被之前未调用过down()的任务调用。 -
这样一来，会出现中断上下文与进程上下文共享数据（
.count）的问题。设想这过程中未禁止中断，在进程上下文拿了自旋锁（.lock）后操作信号量计数，此期间如果被中断打断，并且中断对该信号量调用up()函数，那么up()拿自旋锁的时候会自旋，中断未能返回造成死锁。因此，在操作信号量计数时必须得禁止中断。 -
而大部分时候，在不确定中断状态的情况下，应该用
spin_lock_irqsave()和spin_lock_irqrestore()一类的函数。 -
kernel/locking/semaphore.c
/*
* Copyright (c) 2008 Intel Corporation
* 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>
#include <linux/kernel.h>
#include <linux/export.h>
#include <linux/sched.h>
#include <linux/semaphore.h>
#include <linux/spinlock.h>
#include <linux/ftrace.h>
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 timeout);
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;
raw_spin_lock_irqsave(&sem->lock, flags);
if (likely(sem->count > 0))
sem->count--;
else
__down(sem);
raw_spin_unlock_irqrestore(&sem->lock, flags);
}
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;
int result = 0;
raw_spin_lock_irqsave(&sem->lock, flags);
if (likely(sem->count > 0))
sem->count--;
else
result = __down_interruptible(sem);
raw_spin_unlock_irqrestore(&sem->lock, flags);
return result;
}
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;
int result = 0;
raw_spin_lock_irqsave(&sem->lock, flags);
if (likely(sem->count > 0))
sem->count--;
else
result = __down_killable(sem);
raw_spin_unlock_irqrestore(&sem->lock, flags);
return result;
}
EXPORT_SYMBOL(down_killable);
/**
* down_trylock - try to acquire the semaphore, without waiting
* @sem: the semaphore to be acquired
*
* Try to acquire the semaphore atomically. Returns 0 if the semaphore has
* 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.
*
* Unlike mutex_trylock, this function can be used from interrupt context,
* and the semaphore can be released by any task or interrupt.
*/
int down_trylock(struct semaphore *sem)
{
unsigned long flags;
int count;
raw_spin_lock_irqsave(&sem->lock, flags);
count = sem->count - 1;
if (likely(count >= 0))
sem->count = count;
raw_spin_unlock_irqrestore(&sem->lock, flags);
return (count < 0);
}
EXPORT_SYMBOL(down_trylock);
/**
* down_timeout - acquire the semaphore within a specified time
* @sem: the semaphore to be acquired
* @timeout: 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 timeout)
{
unsigned long flags;
int result = 0;
raw_spin_lock_irqsave(&sem->lock, flags);
if (likely(sem->count > 0))
sem->count--;
else
result = __down_timeout(sem, timeout);
raw_spin_unlock_irqrestore(&sem->lock, flags);
return result;
}
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;
raw_spin_lock_irqsave(&sem->lock, flags);
if (likely(list_empty(&sem->wait_list)))
sem->count++;
else
/*如果在该信号量上的等待队列不为空，那么处于队列中等待的任务会被唤醒*/
__up(sem);
raw_spin_unlock_irqrestore(&sem->lock, flags);
}
EXPORT_SYMBOL(up);
/* Functions for the contended case */
struct semaphore_waiter {
struct list_head list;
struct task_struct *task;
bool up;
};
/*
* Because this function is inlined, the 'state' parameter will be
* constant, and thus optimised away by the compiler. Likewise the
* 'timeout' parameter for the cases without timeouts.
*/
static inline int __sched __down_common(struct semaphore *sem, long state,
long timeout)
{
struct task_struct *task = current;
struct semaphore_waiter waiter;
/*将任务放到因争用这个信号量而等待的链表*/
list_add_tail(&waiter.list, &sem->wait_list);
waiter.task = task;
waiter.up = false;
for (;;) {
/*如果是因为信号的原因被唤醒，返回值得是-EINTR*/
if (signal_pending_state(state, task))
goto interrupted;
if (unlikely(timeout <= 0))
goto timed_out;
__set_task_state(task, state);
/*下面马上要调度了，不能在持有自旋锁时调度，此处需先解锁*/
raw_spin_unlock_irq(&sem->lock);
/*拿不到信号量，会被调度出去*/
timeout = schedule_timeout(timeout);
/*调度完成后再加锁，直至上层函数操作完后再解锁*/
raw_spin_lock_irq(&sem->lock);
if (waiter.up)
return 0;
}
/*因信号或超时的原因苏醒的进程需要多一个移出链表的动作。
对于正常的因信号量释放而唤醒，在up()时已经将进程移出链表了。*/
timed_out:
list_del(&waiter.list);
return -ETIME;
interrupted:
list_del(&waiter.list);
return -EINTR;
}
static noinline void __sched __down(struct semaphore *sem)
{
__down_common(sem, TASK_UNINTERRUPTIBLE, MAX_SCHEDULE_TIMEOUT);
}
static noinline int __sched __down_interruptible(struct semaphore *sem)
{
return __down_common(sem, TASK_INTERRUPTIBLE, MAX_SCHEDULE_TIMEOUT);
}
static noinline int __sched __down_killable(struct semaphore *sem)
{
return __down_common(sem, TASK_KILLABLE, MAX_SCHEDULE_TIMEOUT);
}
static noinline int __sched __down_timeout(struct semaphore *sem, long timeout)
{
return __down_common(sem, TASK_UNINTERRUPTIBLE, timeout);
}
static noinline void __sched __up(struct semaphore *sem)
{ /*将睡眠在这个信号量的等待队列上的第一个等待者取出*/
struct semaphore_waiter *waiter = list_first_entry(&sem->wait_list,
struct semaphore_waiter, list);
/*将等待者移出队列*/
list_del(&waiter->list);
waiter->up = true;
/*唤醒这个等待者*/
wake_up_process(waiter->task);
}