/* * 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/module.h> #include <linux/sched.h> #include <linux/semaphore.h> #include <linux/spinlock.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 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; spin_lock_irqsave(&sem->lock, flags); if (likely(sem->count > 0)) sem->count--; else __down(sem); 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; spin_lock_irqsave(&sem->lock, flags); if (likely(sem->count > 0)) sem->count--; else result = __down_interruptible(sem); 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; spin_lock_irqsave(&sem->lock, flags); if (likely(sem->count > 0)) sem->count--; else result = __down_killable(sem); 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 mutex 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; spin_lock_irqsave(&sem->lock, flags); count = sem->count - 1; if (likely(count >= 0)) sem->count = count; 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 * @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; int result = 0; spin_lock_irqsave(&sem->lock, flags); if (likely(sem->count > 0)) sem->count--; else result = __down_timeout(sem, jiffies); 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; spin_lock_irqsave(&sem->lock, flags); if (likely(list_empty(&sem->wait_list))) sem->count++; else __up(sem); 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; int 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; list_add_tail(&waiter.list, &sem->wait_list); waiter.task = task; waiter.up = 0; for (;;) { if (state == TASK_INTERRUPTIBLE && signal_pending(task)) goto interrupted; if (state == TASK_KILLABLE && fatal_signal_pending(task)) goto interrupted; if (timeout <= 0) goto timed_out; __set_task_state(task, state); spin_unlock_irq(&sem->lock); timeout = schedule_timeout(timeout); spin_lock_irq(&sem->lock); if (waiter.up) return 0; } 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 jiffies) { return __down_common(sem, TASK_UNINTERRUPTIBLE, jiffies); } static noinline void __sched __up(struct semaphore *sem) { struct semaphore_waiter *waiter = list_first_entry(&sem->wait_list, struct semaphore_waiter, list); list_del(&waiter->list); waiter->up = 1; wake_up_process(waiter->task); }