[PATCH] avr32 architecture
[linux-2.6.git] / arch / avr32 / kernel / semaphore.c
1 /*
2  * AVR32 sempahore implementation.
3  *
4  * Copyright (C) 2004-2006 Atmel Corporation
5  *
6  * Based on linux/arch/i386/kernel/semaphore.c
7  *  Copyright (C) 1999 Linus Torvalds
8  *
9  * This program is free software; you can redistribute it and/or modify
10  * it under the terms of the GNU General Public License version 2 as
11  * published by the Free Software Foundation.
12  */
13
14 #include <linux/sched.h>
15 #include <linux/errno.h>
16 #include <linux/module.h>
17
18 #include <asm/semaphore.h>
19 #include <asm/atomic.h>
20
21 /*
22  * Semaphores are implemented using a two-way counter:
23  * The "count" variable is decremented for each process
24  * that tries to acquire the semaphore, while the "sleeping"
25  * variable is a count of such acquires.
26  *
27  * Notably, the inline "up()" and "down()" functions can
28  * efficiently test if they need to do any extra work (up
29  * needs to do something only if count was negative before
30  * the increment operation.
31  *
32  * "sleeping" and the contention routine ordering is protected
33  * by the spinlock in the semaphore's waitqueue head.
34  *
35  * Note that these functions are only called when there is
36  * contention on the lock, and as such all this is the
37  * "non-critical" part of the whole semaphore business. The
38  * critical part is the inline stuff in <asm/semaphore.h>
39  * where we want to avoid any extra jumps and calls.
40  */
41
42 /*
43  * Logic:
44  *  - only on a boundary condition do we need to care. When we go
45  *    from a negative count to a non-negative, we wake people up.
46  *  - when we go from a non-negative count to a negative do we
47  *    (a) synchronize with the "sleeper" count and (b) make sure
48  *    that we're on the wakeup list before we synchronize so that
49  *    we cannot lose wakeup events.
50  */
51
52 void __up(struct semaphore *sem)
53 {
54         wake_up(&sem->wait);
55 }
56 EXPORT_SYMBOL(__up);
57
58 void __sched __down(struct semaphore *sem)
59 {
60         struct task_struct *tsk = current;
61         DECLARE_WAITQUEUE(wait, tsk);
62         unsigned long flags;
63
64         tsk->state = TASK_UNINTERRUPTIBLE;
65         spin_lock_irqsave(&sem->wait.lock, flags);
66         add_wait_queue_exclusive_locked(&sem->wait, &wait);
67
68         sem->sleepers++;
69         for (;;) {
70                 int sleepers = sem->sleepers;
71
72                 /*
73                  * Add "everybody else" into it. They aren't
74                  * playing, because we own the spinlock in
75                  * the wait_queue_head.
76                  */
77                 if (atomic_add_return(sleepers - 1, &sem->count) >= 0) {
78                         sem->sleepers = 0;
79                         break;
80                 }
81                 sem->sleepers = 1;      /* us - see -1 above */
82                 spin_unlock_irqrestore(&sem->wait.lock, flags);
83
84                 schedule();
85
86                 spin_lock_irqsave(&sem->wait.lock, flags);
87                 tsk->state = TASK_UNINTERRUPTIBLE;
88         }
89         remove_wait_queue_locked(&sem->wait, &wait);
90         wake_up_locked(&sem->wait);
91         spin_unlock_irqrestore(&sem->wait.lock, flags);
92         tsk->state = TASK_RUNNING;
93 }
94 EXPORT_SYMBOL(__down);
95
96 int __sched __down_interruptible(struct semaphore *sem)
97 {
98         int retval = 0;
99         struct task_struct *tsk = current;
100         DECLARE_WAITQUEUE(wait, tsk);
101         unsigned long flags;
102
103         tsk->state = TASK_INTERRUPTIBLE;
104         spin_lock_irqsave(&sem->wait.lock, flags);
105         add_wait_queue_exclusive_locked(&sem->wait, &wait);
106
107         sem->sleepers++;
108         for (;;) {
109                 int sleepers = sem->sleepers;
110
111                 /*
112                  * With signals pending, this turns into the trylock
113                  * failure case - we won't be sleeping, and we can't
114                  * get the lock as it has contention. Just correct the
115                  * count and exit.
116                  */
117                 if (signal_pending(current)) {
118                         retval = -EINTR;
119                         sem->sleepers = 0;
120                         atomic_add(sleepers, &sem->count);
121                         break;
122                 }
123
124                 /*
125                  * Add "everybody else" into it. They aren't
126                  * playing, because we own the spinlock in
127                  * the wait_queue_head.
128                  */
129                 if (atomic_add_return(sleepers - 1, &sem->count) >= 0) {
130                         sem->sleepers = 0;
131                         break;
132                 }
133                 sem->sleepers = 1;      /* us - see -1 above */
134                 spin_unlock_irqrestore(&sem->wait.lock, flags);
135
136                 schedule();
137
138                 spin_lock_irqsave(&sem->wait.lock, flags);
139                 tsk->state = TASK_INTERRUPTIBLE;
140         }
141         remove_wait_queue_locked(&sem->wait, &wait);
142         wake_up_locked(&sem->wait);
143         spin_unlock_irqrestore(&sem->wait.lock, flags);
144
145         tsk->state = TASK_RUNNING;
146         return retval;
147 }
148 EXPORT_SYMBOL(__down_interruptible);