Linux内核读写信号量 rwsem实现分析.doc

读写信号量 信号量根据用途不同，可以区分读和写两种操作方式。为提高信号量的效率引入rwsem，此锁使用的机会比较少，适用于读频繁的情况。头文件，类型struct rw_semaphore，其必须在运行时进行显示的初始化：void init_rwsem(struct rw_semaphore* sem);对于只读访问，使用如下PV：void down_read(struct rw_semaphore* sem);int down_read_trylock(struct rw_semaphore* sem);/成功返回非零，其他返回零（特殊）void up_read(struct rw_semaphore* sem);对于写入分别是：down_write, down_write_trylock, up_writerwsem允许一个写入者或无限个读者拥有该信号量。在2.6.21内核rwsem的实现中，所有读者和写者排队处理，处理完写者前面的所有读者后才处理到写者。在rwsem-spinlock.c中down_read实现如下：/* get a read lock on the semaphore*/void fastcall _sched _down_read(struct rw_semaphore *sem)struct rwsem_waiter waiter;struct task_struct *tsk;spin_lock_irq(&sem-wait_lock); / 没有写者的情况下直接获取读者锁if (sem-activity = 0 & list_empty(&sem-wait_list) /* granted */sem-activity+;spin_unlock_irq(&sem-wait_lock);goto out; / 否则有写者的情况下，加入到排队队列中tsk = current;set_task_state(tsk, TASK_UNINTERRUPTIBLE);/ 进程不可中断/* set up my own style of waitqueue */waiter.task = tsk;waiter.flags = RWSEM_WAITING_FOR_READ;get_task_struct(tsk);list_add_tail(&waiter.list, &sem-wait_list);/ 加入到排队队列中/* we dont need to touch the semaphore struct anymore */spin_unlock_irq(&sem-wait_lock);/* wait to be given the lock */for (;) / task有效的时候，继续调度，在up中会被设置成NULL，进而去调度if (!waiter.task)break;schedule();set_task_state(tsk, TASK_UNINTERRUPTIBLE);tsk-state = TASK_RUNNING;out:;/* release a read lock on the semaphore*/void fastcall _up_read(struct rw_semaphore *sem)unsigned long flags;spin_lock_irqsave(&sem-wait_lock, flags); / 释放读者锁的时候，若wait_list不为空的时候，证明等待队列中存在写者锁请求if (-sem-activity = 0 & !list_empty(&sem-wait_list)sem = _rwsem_wake_one_writer(sem);/ 此函数下面分析spin_unlock_irqrestore(&sem-wait_lock, flags);down_write函数会直接调用以下函数/* get a write lock on the semaphore* - we increment the waiting count anyway to indicate an exclusive lock*/void fastcall _sched _down_write_nested(struct rw_semaphore *sem, int subclass)struct rwsem_waiter waiter;struct task_struct *tsk;spin_lock_irq(&sem-wait_lock);/ activity = 0且队列中没有等待时，此时没有读者和写者，直接获取写者锁。if (sem-activity = 0 & list_empty(&sem-wait_list) /* granted */sem-activity = -1;spin_unlock_irq(&sem-wait_lock);goto out;/ 否则去排队tsk = current;set_task_state(tsk, TASK_UNINTERRUPTIBLE);/* set up my own style of waitqueue */waiter.task = tsk;waiter.flags = RWSEM_WAITING_FOR_WRITE;get_task_struct(tsk);list_add_tail(&waiter.list, &sem-wait_list);/* we dont need to touch the semaphore struct anymore */spin_unlock_irq(&sem-wait_lock);/* wait to be given the lock */for (;) if (!waiter.task)break;schedule();set_task_state(tsk, TASK_UNINTERRUPTIBLE);tsk-state = TASK_RUNNING;out:;down_write函数会被_rwsem_wake_one_writer唤醒。/* wake a single writer*/static inline struct rw_semaphore *_rwsem_wake_one_writer(struct rw_semaphore *sem)struct rwsem_waiter *waiter;struct task_struct *tsk;sem-activity = -1; / 表示当前没有写者锁waiter = list_entry(sem-wait_list.next, struct rwsem_waiter, list);list_del(&waiter-list);/ 设置waiter-task为NULL，并唤醒进程tsk = waiter-task;smp_mb();waiter-task = NULL;wake_up_process(tsk);put_task_struct(tsk);return sem;Up_write函数如下：/* release a write lock on the semaphore*/void fastcall _up_write(struct rw_semaphore *sem)unsigned long flags;spin_lock_irqsave(&sem-wait_lock, flags);sem-activity = 0; / 没有读者和写者/ 如果队列不为空，则唤醒，第二个参数1的意思是所有的读者和写者if (!list_empty(&sem-wait_list)sem = _rwsem_do_wake(sem, 1);spin_unlock_irqrestore(&sem-wait_lock, flags);/* handle the lock release when processes blocked on it that can now run* - if we come here, then:* - the active count _reached_ zero* - the waiting count is non-zero* - the spinlock must be held by the caller* - woken process blocks are discarded from the list after having task zeroed* - writers are only woken if wakewrite is non-zero */static inline struct rw_semaphore * _rwsem_do_wake(struct rw_semaphore *sem, int wakewrite)struct rwsem_waiter *waiter;struct task_struct *tsk;int woken;waiter = list_entry(sem-wait_list.next, struct rwsem_waiter, list);/ 若wakewrite为0则只唤醒读者，为1时唤醒所有写者和读者if (!wakewrite) if (waiter-flags & RWSEM_WAITING_FOR_WRITE)goto out;goto dont_wake_writers;/ 唤醒写者，list的第一个元素if (waiter-flags & RWSEM_WAITING_FOR_WRITE) sem-activity = -1;list_del(&waiter-list);tsk = waiter-task;/* Dont touch waiter after -task has been NULLed */smp_mb();waiter-task = NULL;wake_up_process(tsk);put_task_struct(tsk);goto out;/ 唤醒所有的读者，activity值直接加到位/* grant an infinite number of read locks to the front of the queue */dont_wake_writers:woken = 0;while (waiter-flags & RWSEM_WAITING_FOR_READ) struct list_head *next = waiter-list.next;list_del(&waiter-list);ts