Linux 2.6.25-rc4
[linux-2.6/next.git] / arch / ia64 / kernel / semaphore.c
blob2724ef3fbae2e78e9d4954c9b7cfe6ba91c37194
1 /*
2 * IA-64 semaphore implementation (derived from x86 version).
4 * Copyright (C) 1999-2000, 2002 Hewlett-Packard Co
5 * David Mosberger-Tang <davidm@hpl.hp.com>
6 */
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.
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.
18 * "sleeping" and the contention routine ordering is protected
19 * by the spinlock in the semaphore's waitqueue head.
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.
26 #include <linux/sched.h>
27 #include <linux/init.h>
29 #include <asm/errno.h>
30 #include <asm/semaphore.h>
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 "sleepers" count and (b) make sure
38 * that we're on the wakeup list before we synchronize so that
39 * we cannot lose wakeup events.
42 void
43 __up (struct semaphore *sem)
45 wake_up(&sem->wait);
48 void __sched __down (struct semaphore *sem)
50 struct task_struct *tsk = current;
51 DECLARE_WAITQUEUE(wait, tsk);
52 unsigned long flags;
54 tsk->state = TASK_UNINTERRUPTIBLE;
55 spin_lock_irqsave(&sem->wait.lock, flags);
56 add_wait_queue_exclusive_locked(&sem->wait, &wait);
58 sem->sleepers++;
59 for (;;) {
60 int sleepers = sem->sleepers;
63 * Add "everybody else" into it. They aren't
64 * playing, because we own the spinlock in
65 * the wait_queue_head.
67 if (!atomic_add_negative(sleepers - 1, &sem->count)) {
68 sem->sleepers = 0;
69 break;
71 sem->sleepers = 1; /* us - see -1 above */
72 spin_unlock_irqrestore(&sem->wait.lock, flags);
74 schedule();
76 spin_lock_irqsave(&sem->wait.lock, flags);
77 tsk->state = TASK_UNINTERRUPTIBLE;
79 remove_wait_queue_locked(&sem->wait, &wait);
80 wake_up_locked(&sem->wait);
81 spin_unlock_irqrestore(&sem->wait.lock, flags);
82 tsk->state = TASK_RUNNING;
85 int __sched __down_interruptible (struct semaphore * sem)
87 int retval = 0;
88 struct task_struct *tsk = current;
89 DECLARE_WAITQUEUE(wait, tsk);
90 unsigned long flags;
92 tsk->state = TASK_INTERRUPTIBLE;
93 spin_lock_irqsave(&sem->wait.lock, flags);
94 add_wait_queue_exclusive_locked(&sem->wait, &wait);
96 sem->sleepers ++;
97 for (;;) {
98 int sleepers = sem->sleepers;
101 * With signals pending, this turns into
102 * the trylock failure case - we won't be
103 * sleeping, and we* can't get the lock as
104 * it has contention. Just correct the count
105 * and exit.
107 if (signal_pending(current)) {
108 retval = -EINTR;
109 sem->sleepers = 0;
110 atomic_add(sleepers, &sem->count);
111 break;
115 * Add "everybody else" into it. They aren't
116 * playing, because we own the spinlock in
117 * wait_queue_head. The "-1" is because we're
118 * still hoping to get the semaphore.
120 if (!atomic_add_negative(sleepers - 1, &sem->count)) {
121 sem->sleepers = 0;
122 break;
124 sem->sleepers = 1; /* us - see -1 above */
125 spin_unlock_irqrestore(&sem->wait.lock, flags);
127 schedule();
129 spin_lock_irqsave(&sem->wait.lock, flags);
130 tsk->state = TASK_INTERRUPTIBLE;
132 remove_wait_queue_locked(&sem->wait, &wait);
133 wake_up_locked(&sem->wait);
134 spin_unlock_irqrestore(&sem->wait.lock, flags);
136 tsk->state = TASK_RUNNING;
137 return retval;
141 * Trylock failed - make sure we correct for having decremented the
142 * count.
145 __down_trylock (struct semaphore *sem)
147 unsigned long flags;
148 int sleepers;
150 spin_lock_irqsave(&sem->wait.lock, flags);
151 sleepers = sem->sleepers + 1;
152 sem->sleepers = 0;
155 * Add "everybody else" and us into it. They aren't
156 * playing, because we own the spinlock in the
157 * wait_queue_head.
159 if (!atomic_add_negative(sleepers, &sem->count)) {
160 wake_up_locked(&sem->wait);
163 spin_unlock_irqrestore(&sem->wait.lock, flags);
164 return 1;