Merge git://git.kernel.org/pub/scm/linux/kernel/git/sam/kbuild-2.6.18
[linux-2.6] / arch / sparc / kernel / semaphore.c
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 }