File: /usr/src/linux/arch/ia64/kernel/semaphore.c
1 /*
2 * IA-64 semaphore implementation (derived from x86 version).
3 *
4 * Copyright (C) 1999-2000 Hewlett-Packard Co
5 * Copyright (C) 1999-2000 David Mosberger-Tang <davidm@hpl.hp.com>
6 */
7
8 /*
9 * Semaphores are implemented using a two-way counter: The "count"
10 * variable is decremented for each process that tries to acquire the
11 * semaphore, while the "sleepers" variable is a count of such
12 * acquires.
13 *
14 * Notably, the inline "up()" and "down()" functions can efficiently
15 * test if they need to do any extra work (up needs to do something
16 * only if count was negative before the increment operation.
17 *
18 * "sleepers" and the contention routine ordering is protected by the
19 * semaphore spinlock.
20 *
21 * Note that these functions are only called when there is contention
22 * on the lock, and as such all this is the "non-critical" part of the
23 * whole semaphore business. The critical part is the inline stuff in
24 * <asm/semaphore.h> where we want to avoid any extra jumps and calls.
25 */
26 #include <linux/sched.h>
27
28 #include <asm/semaphore.h>
29
30 /*
31 * Logic:
32 * - Only on a boundary condition do we need to care. When we go
33 * from a negative count to a non-negative, we wake people up.
34 * - When we go from a non-negative count to a negative do we
35 * (a) synchronize with the "sleepers" count and (b) make sure
36 * that we're on the wakeup list before we synchronize so that
37 * we cannot lose wakeup events.
38 */
39
40 void
41 __up (struct semaphore *sem)
42 {
43 wake_up(&sem->wait);
44 }
45
46 static spinlock_t semaphore_lock = SPIN_LOCK_UNLOCKED;
47
48 void
49 __down (struct semaphore *sem)
50 {
51 struct task_struct *tsk = current;
52 DECLARE_WAITQUEUE(wait, tsk);
53 tsk->state = TASK_UNINTERRUPTIBLE;
54 add_wait_queue_exclusive(&sem->wait, &wait);
55
56 spin_lock_irq(&semaphore_lock);
57 sem->sleepers++;
58 for (;;) {
59 int sleepers = sem->sleepers;
60
61 /*
62 * Add "everybody else" into it. They aren't
63 * playing, because we own the spinlock.
64 */
65 if (!atomic_add_negative(sleepers - 1, &sem->count)) {
66 sem->sleepers = 0;
67 break;
68 }
69 sem->sleepers = 1; /* us - see -1 above */
70 spin_unlock_irq(&semaphore_lock);
71
72 schedule();
73 tsk->state = TASK_UNINTERRUPTIBLE;
74 spin_lock_irq(&semaphore_lock);
75 }
76 spin_unlock_irq(&semaphore_lock);
77 remove_wait_queue(&sem->wait, &wait);
78 tsk->state = TASK_RUNNING;
79 wake_up(&sem->wait);
80 }
81
82 int
83 __down_interruptible (struct semaphore * sem)
84 {
85 int retval = 0;
86 struct task_struct *tsk = current;
87 DECLARE_WAITQUEUE(wait, tsk);
88 tsk->state = TASK_INTERRUPTIBLE;
89 add_wait_queue_exclusive(&sem->wait, &wait);
90
91 spin_lock_irq(&semaphore_lock);
92 sem->sleepers ++;
93 for (;;) {
94 int sleepers = sem->sleepers;
95
96 /*
97 * With signals pending, this turns into
98 * the trylock failure case - we won't be
99 * sleeping, and we* can't get the lock as
100 * it has contention. Just correct the count
101 * and exit.
102 */
103 if (signal_pending(current)) {
104 retval = -EINTR;
105 sem->sleepers = 0;
106 atomic_add(sleepers, &sem->count);
107 break;
108 }
109
110 /*
111 * Add "everybody else" into it. They aren't
112 * playing, because we own the spinlock. The
113 * "-1" is because we're still hoping to get
114 * the lock.
115 */
116 if (!atomic_add_negative(sleepers - 1, &sem->count)) {
117 sem->sleepers = 0;
118 break;
119 }
120 sem->sleepers = 1; /* us - see -1 above */
121 spin_unlock_irq(&semaphore_lock);
122
123 schedule();
124 tsk->state = TASK_INTERRUPTIBLE;
125 spin_lock_irq(&semaphore_lock);
126 }
127 spin_unlock_irq(&semaphore_lock);
128 tsk->state = TASK_RUNNING;
129 remove_wait_queue(&sem->wait, &wait);
130 wake_up(&sem->wait);
131 return retval;
132 }
133
134 /*
135 * Trylock failed - make sure we correct for having decremented the
136 * count.
137 */
138 int
139 __down_trylock (struct semaphore *sem)
140 {
141 unsigned long flags;
142 int sleepers;
143
144 spin_lock_irqsave(&semaphore_lock, flags);
145 sleepers = sem->sleepers + 1;
146 sem->sleepers = 0;
147
148 /*
149 * Add "everybody else" and us into it. They aren't
150 * playing, because we own the spinlock.
151 */
152 if (!atomic_add_negative(sleepers, &sem->count))
153 wake_up(&sem->wait);
154
155 spin_unlock_irqrestore(&semaphore_lock, flags);
156 return 1;
157 }
158