1 /* $Id: semaphore.c,v 1.7 2001/04/18 21:06:05 davem Exp $ */
2
3 /* sparc32 semaphore implementation, based on i386 version */
4
5 #include <linux/sched.h>
6 #include <linux/errno.h>
7 #include <linux/init.h>
8
9 #include <asm/semaphore.h>
10
11 /*
12 * Semaphores are implemented using a two-way counter:
13 * The "count" variable is decremented for each process
14 * that tries to acquire the semaphore, while the "sleeping"
15 * variable is a count of such acquires.
16 *
17 * Notably, the inline "up()" and "down()" functions can
18 * efficiently test if they need to do any extra work (up
19 * needs to do something only if count was negative before
20 * the increment operation.
21 *
22 * "sleeping" and the contention routine ordering is
23 * protected by the semaphore spinlock.
24 *
25 * Note that these functions are only called when there is
26 * contention on the lock, and as such all this is the
27 * "non-critical" part of the whole semaphore business. The
28 * critical part is the inline stuff in <asm/semaphore.h>
29 * where we want to avoid any extra jumps and calls.
30 */
31
32 /*
33 * Logic:
34 * - only on a boundary condition do we need to care. When we go
35 * from a negative count to a non-negative, we wake people up.
36 * - when we go from a non-negative count to a negative do we
37 * (a) synchronize with the "sleeper" count and (b) make sure
38 * that we're on the wakeup list before we synchronize so that
39 * we cannot lose wakeup events.
40 */
41
42 void __up(struct semaphore *sem)
43 {
44 wake_up(&sem->wait);
45 }
46
47 static DEFINE_SPINLOCK(semaphore_lock);
48
49 void __sched __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 (!atomic24_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 __sched __down_interruptible(struct semaphore * sem)
83 {
84 int retval = 0;
85 struct task_struct *tsk = current;
86 DECLARE_WAITQUEUE(wait, tsk);
87 tsk->state = TASK_INTERRUPTIBLE;
88 add_wait_queue_exclusive(&sem->wait, &wait);
89
90 spin_lock_irq(&semaphore_lock);
91 sem->sleepers ++;
92 for (;;) {
93 int sleepers = sem->sleepers;
94
95 /*
96 * With signals pending, this turns into
97 * the trylock failure case - we won't be
98 * sleeping, and we* can't get the lock as
99 * it has contention. Just correct the count
100 * and exit.
101 */
102 if (signal_pending(current)) {
103 retval = -EINTR;
104 sem->sleepers = 0;
105 atomic24_add(sleepers, &sem->count);
106 break;
107 }
108
109 /*
110 * Add "everybody else" into it. They aren't
111 * playing, because we own the spinlock. The
112 * "-1" is because we're still hoping to get
113 * the lock.
114 */
115 if (!atomic24_add_negative(sleepers - 1, &sem->count)) {
116 sem->sleepers = 0;
117 break;
118 }
119 sem->sleepers = 1; /* us - see -1 above */
120 spin_unlock_irq(&semaphore_lock);
121
122 schedule();
123 tsk->state = TASK_INTERRUPTIBLE;
124 spin_lock_irq(&semaphore_lock);
125 }
126 spin_unlock_irq(&semaphore_lock);
127 tsk->state = TASK_RUNNING;
128 remove_wait_queue(&sem->wait, &wait);
129 wake_up(&sem->wait);
130 return retval;
131 }
132
133 /*
134 * Trylock failed - make sure we correct for
135 * having decremented the count.
136 */
137 int __down_trylock(struct semaphore * sem)
138 {
139 int sleepers;
140 unsigned long flags;
141
142 spin_lock_irqsave(&semaphore_lock, flags);
143 sleepers = sem->sleepers + 1;
144 sem->sleepers = 0;
145
146 /*
147 * Add "everybody else" and us into it. They aren't
148 * playing, because we own the spinlock.
149 */
150 if (!atomic24_add_negative(sleepers, &sem->count))
151 wake_up(&sem->wait);
152
153 spin_unlock_irqrestore(&semaphore_lock, flags);
154 return 1;
155 }
156
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