2 * Copyright (c) 2008 Intel Corporation
3 * Author: Matthew Wilcox <willy@linux.intel.com>
5 * Distributed under the terms of the GNU GPL, version 2
8 #include <linux/compiler.h>
9 #include <linux/kernel.h>
10 #include <linux/module.h>
11 #include <linux/sched.h>
12 #include <linux/semaphore.h>
13 #include <linux/spinlock.h>
16 * Some notes on the implementation:
18 * down_trylock() and up() can be called from interrupt context.
19 * So we have to disable interrupts when taking the lock.
21 * The ->count variable, if positive, defines how many more tasks can
22 * acquire the semaphore. If negative, it represents how many tasks are
23 * waiting on the semaphore (*). If zero, no tasks are waiting, and no more
24 * tasks can acquire the semaphore.
26 * (*) Except for the window between one task calling up() and the task
27 * sleeping in a __down_common() waking up. In order to avoid a third task
28 * coming in and stealing the second task's wakeup, we leave the ->count
29 * negative. If we have a more complex situation, the ->count may become
30 * zero or negative (eg a semaphore with count = 2, three tasks attempt to
31 * acquire it, one sleeps, two finish and call up(), the second task to call
32 * up() notices that the list is empty and just increments count).
35 static noinline void __down(struct semaphore *sem);
36 static noinline int __down_interruptible(struct semaphore *sem);
37 static noinline void __up(struct semaphore *sem);
39 void down(struct semaphore *sem)
43 spin_lock_irqsave(&sem->lock, flags);
44 if (unlikely(sem->count-- <= 0))
46 spin_unlock_irqrestore(&sem->lock, flags);
50 int down_interruptible(struct semaphore *sem)
55 spin_lock_irqsave(&sem->lock, flags);
56 if (unlikely(sem->count-- <= 0))
57 result = __down_interruptible(sem);
58 spin_unlock_irqrestore(&sem->lock, flags);
62 EXPORT_SYMBOL(down_interruptible);
65 * down_trylock - try to acquire the semaphore, without waiting
66 * @sem: the semaphore to be acquired
68 * Try to acquire the semaphore atomically. Returns 0 if the mutex has
69 * been acquired successfully and 1 if it is contended.
71 * NOTE: This return value is inverted from both spin_trylock and
72 * mutex_trylock! Be careful about this when converting code.
74 * Unlike mutex_trylock, this function can be used from interrupt context,
75 * and the semaphore can be released by any task or interrupt.
77 int down_trylock(struct semaphore *sem)
82 spin_lock_irqsave(&sem->lock, flags);
83 count = sem->count - 1;
84 if (likely(count >= 0))
86 spin_unlock_irqrestore(&sem->lock, flags);
90 EXPORT_SYMBOL(down_trylock);
92 void up(struct semaphore *sem)
96 spin_lock_irqsave(&sem->lock, flags);
97 if (likely(sem->count >= 0))
101 spin_unlock_irqrestore(&sem->lock, flags);
105 /* Functions for the contended case */
107 struct semaphore_waiter {
108 struct list_head list;
109 struct task_struct *task;
114 * Wake up a process waiting on a semaphore. We need to call this from both
115 * __up and __down_common as it's possible to race a task into the semaphore
116 * if it comes in at just the right time between two tasks calling up() and
117 * a third task waking up. This function assumes the wait_list is already
118 * checked for being non-empty.
120 static noinline void __sched __up_down_common(struct semaphore *sem)
122 struct semaphore_waiter *waiter = list_first_entry(&sem->wait_list,
123 struct semaphore_waiter, list);
124 list_del(&waiter->list);
126 wake_up_process(waiter->task);
130 * Because this function is inlined, the 'state' parameter will be constant,
131 * and thus optimised away by the compiler.
133 static inline int __sched __down_common(struct semaphore *sem, long state)
136 struct task_struct *task = current;
137 struct semaphore_waiter waiter;
139 list_add_tail(&waiter.list, &sem->wait_list);
144 if (state == TASK_INTERRUPTIBLE && signal_pending(task))
146 __set_task_state(task, state);
147 spin_unlock_irq(&sem->lock);
149 spin_lock_irq(&sem->lock);
155 list_del(&waiter.list);
159 * Account for the process which woke us up. For the case where
160 * we're interrupted, we need to increment the count on our own
161 * behalf. I don't believe we can hit the case where the
162 * sem->count hits zero, *and* there's a second task sleeping,
163 * but it doesn't hurt, that's not a commonly exercised path and
164 * it's not a performance path either.
166 if (unlikely((++sem->count >= 0) && !list_empty(&sem->wait_list)))
167 __up_down_common(sem);
171 static noinline void __sched __down(struct semaphore *sem)
173 __down_common(sem, TASK_UNINTERRUPTIBLE);
176 static noinline int __sched __down_interruptible(struct semaphore *sem)
178 return __down_common(sem, TASK_INTERRUPTIBLE);
181 static noinline void __sched __up(struct semaphore *sem)
183 if (unlikely(list_empty(&sem->wait_list)))
186 __up_down_common(sem);