1/*
2 * IA-64 semaphore implementation (derived from x86 version).
3 *
4 * Copyright (C) 1999-2000, 2002 Hewlett-Packard Co
5 *	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 * "sleeping" and the contention routine ordering is protected
19 * by the spinlock in the semaphore's waitqueue head.
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#include <linux/init.h>
28
29#include <asm/errno.h>
30#include <asm/semaphore.h>
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 "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.
40 */
41
42void
43__up (struct semaphore *sem)
44{
45	wake_up(&sem->wait);
46}
47
48void __sched __down (struct semaphore *sem)
49{
50	struct task_struct *tsk = current;
51	DECLARE_WAITQUEUE(wait, tsk);
52	unsigned long flags;
53
54	tsk->state = TASK_UNINTERRUPTIBLE;
55	spin_lock_irqsave(&sem->wait.lock, flags);
56	add_wait_queue_exclusive_locked(&sem->wait, &wait);
57
58	sem->sleepers++;
59	for (;;) {
60		int sleepers = sem->sleepers;
61
62		/*
63		 * Add "everybody else" into it. They aren't
64		 * playing, because we own the spinlock in
65		 * the wait_queue_head.
66		 */
67		if (!atomic_add_negative(sleepers - 1, &sem->count)) {
68			sem->sleepers = 0;
69			break;
70		}
71		sem->sleepers = 1;	/* us - see -1 above */
72		spin_unlock_irqrestore(&sem->wait.lock, flags);
73
74		schedule();
75
76		spin_lock_irqsave(&sem->wait.lock, flags);
77		tsk->state = TASK_UNINTERRUPTIBLE;
78	}
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;
83}
84
85int __sched __down_interruptible (struct semaphore * sem)
86{
87	int retval = 0;
88	struct task_struct *tsk = current;
89	DECLARE_WAITQUEUE(wait, tsk);
90	unsigned long flags;
91
92	tsk->state = TASK_INTERRUPTIBLE;
93	spin_lock_irqsave(&sem->wait.lock, flags);
94	add_wait_queue_exclusive_locked(&sem->wait, &wait);
95
96	sem->sleepers ++;
97	for (;;) {
98		int sleepers = sem->sleepers;
99
100		/*
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.
106		 */
107		if (signal_pending(current)) {
108			retval = -EINTR;
109			sem->sleepers = 0;
110			atomic_add(sleepers, &sem->count);
111			break;
112		}
113
114		/*
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.
119		 */
120		if (!atomic_add_negative(sleepers - 1, &sem->count)) {
121			sem->sleepers = 0;
122			break;
123		}
124		sem->sleepers = 1;	/* us - see -1 above */
125		spin_unlock_irqrestore(&sem->wait.lock, flags);
126
127		schedule();
128
129		spin_lock_irqsave(&sem->wait.lock, flags);
130		tsk->state = TASK_INTERRUPTIBLE;
131	}
132	remove_wait_queue_locked(&sem->wait, &wait);
133	wake_up_locked(&sem->wait);
134	spin_unlock_irqrestore(&sem->wait.lock, flags);
135
136	tsk->state = TASK_RUNNING;
137	return retval;
138}
139
140/*
141 * Trylock failed - make sure we correct for having decremented the
142 * count.
143 */
144int
145__down_trylock (struct semaphore *sem)
146{
147	unsigned long flags;
148	int sleepers;
149
150	spin_lock_irqsave(&sem->wait.lock, flags);
151	sleepers = sem->sleepers + 1;
152	sem->sleepers = 0;
153
154	/*
155	 * Add "everybody else" and us into it. They aren't
156	 * playing, because we own the spinlock in the
157	 * wait_queue_head.
158	 */
159	if (!atomic_add_negative(sleepers, &sem->count)) {
160		wake_up_locked(&sem->wait);
161	}
162
163	spin_unlock_irqrestore(&sem->wait.lock, flags);
164	return 1;
165}
166