Linux内核信号量(semaphore)使用与源码分析
https://blog.csdn.net/Auris/article/details/107404962
一. 在Linux内核驱动中使用信号量(semaphore)常规操作步骤:
[0]. 定义信号量结构体变量;
struct semaphore sem;
[1]. 初始化信号量变量
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void sema_init(struct semaphore *sem, int n);
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eg. sema_init(&sem, 1);
[2]. 获取信号量:
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void down(struct semaphore *sem); // 获取信号量, 资源不足则睡眠等待
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int down_trylock(struct semaphore* sem); // 试图获取信号量, 如果没有则直接返回 不睡眠
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int down_interruptible(struct semaphore* sem); // 获取信号量, 可以被信号打断;
[3]. 释放信号量:
void up(struct semaphore* sem); // 释放信号量, 唤醒信号等待队列中的第一个等待进程
二. 重要的数据结构体:
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struct semaphore {
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raw_spinlock_t lock;
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unsigned int count;
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struct list_head wait_list;
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};
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struct semaphore_waiter {
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struct task_struct *task;
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bool up;
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struct list_head list;
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};
三. down操作源码分析, 以down函数为例:
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void down(struct semaphore *sem)
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{
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unsigned long flags;
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raw_spin_lock_irqsave(&sem->lock, flags);
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if (likely(sem->count > 0))
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sem->count--;
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else
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__down(sem);
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raw_spin_unlock_irqrestore(&sem->lock, flags);
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}
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static noinline void __sched __down(struct semaphore *sem)
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{
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__down_common(sem, TASK_UNINTERRUPTIBLE, MAX_SCHEDULE_TIMEOUT);
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}
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static inline int __sched __down_common(struct semaphore *sem, long state, long timeout)
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{
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struct task_struct *task = current;
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struct semaphore_waiter waiter;
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list_add_tail(&waiter.list, &sem->wait_list);
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waiter.task = task;
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waiter.up = false;
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for (;;) {
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if (signal_pending_state(state, task))
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goto interrupted;
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if (unlikely(timeout <= 0))
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goto timed_out;
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__set_task_state(task, state);
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raw_spin_unlock_irq(&sem->lock);
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timeout = schedule_timeout(timeout);
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raw_spin_lock_irq(&sem->lock);
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if (waiter.up)
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return 0;
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}
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timed_out:
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list_del(&waiter.list);
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return -ETIME;
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interrupted:
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list_del(&waiter.list);
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return -EINTR;
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}
四. up操作源码分析, 以up函数为例:
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void up(struct semaphore *sem)
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{
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unsigned long flags;
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raw_spin_lock_irqsave(&sem->lock, flags);
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if (likely(list_empty(&sem->wait_list)))
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sem->count++;
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else
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__up(sem);
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raw_spin_unlock_irqrestore(&sem->lock, flags);
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}
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static noinline void __sched __up(struct semaphore *sem)
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{
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struct semaphore_waiter *waiter = list_first_entry(&sem->wait_list,
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struct semaphore_waiter, list);
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list_del(&waiter->list);
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waiter->up = true;
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wake_up_process(waiter->task);
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}
五. 流程分析与注意点:
[0]. sem->count的数值>=0, 如果count>0, 说明当前信号量没有被占用, 可以获取;
如果count<=0(==0), 需要根据sem->list来添加等待任务或者唤醒任务;