1/*
2 *  linux/arch/S390/kernel/semaphore.c
3 *
4 *  S390 version
5 *    Copyright (C) 1998-2000 IBM Corporation
6 *    Author(s): Martin Schwidefsky
7 *
8 *  Derived from "linux/arch/i386/kernel/semaphore.c
9 *    Copyright (C) 1999, Linus Torvalds
10 *
11 */
12#include <linux/sched.h>
13
14#include <asm/semaphore.h>
15
16/*
17 * Semaphores are implemented using a two-way counter:
18 * The "count" variable is decremented for each process
19 * that tries to acquire the semaphore, while the "sleeping"
20 * variable is a count of such acquires.
21 *
22 * Notably, the inline "up()" and "down()" functions can
23 * efficiently test if they need to do any extra work (up
24 * needs to do something only if count was negative before
25 * the increment operation.
26 *
27 * "sleeping" and the contention routine ordering is
28 * protected by the semaphore spinlock.
29 *
30 * Note that these functions are only called when there is
31 * contention on the lock, and as such all this is the
32 * "non-critical" part of the whole semaphore business. The
33 * critical part is the inline stuff in <asm/semaphore.h>
34 * where we want to avoid any extra jumps and calls.
35 */
36
37/*
38 * Logic:
39 *  - only on a boundary condition do we need to care. When we go
40 *    from a negative count to a non-negative, we wake people up.
41 *  - when we go from a non-negative count to a negative do we
42 *    (a) synchronize with the "sleeper" count and (b) make sure
43 *    that we're on the wakeup list before we synchronize so that
44 *    we cannot lose wakeup events.
45 */
46
47void __up(struct semaphore *sem)
48{
49	wake_up(&sem->wait);
50}
51
52static spinlock_t semaphore_lock = SPIN_LOCK_UNLOCKED;
53
54void __down(struct semaphore * sem)
55{
56	struct task_struct *tsk = current;
57	DECLARE_WAITQUEUE(wait, tsk);
58	tsk->state = TASK_UNINTERRUPTIBLE;
59	add_wait_queue_exclusive(&sem->wait, &wait);
60
61	spin_lock_irq(&semaphore_lock);
62	sem->sleepers++;
63	for (;;) {
64		int sleepers = sem->sleepers;
65
66		/*
67		 * Add "everybody else" into it. They aren't
68		 * playing, because we own the spinlock.
69		 */
70		if (!atomic_add_negative(sleepers - 1, &sem->count)) {
71			sem->sleepers = 0;
72			break;
73		}
74		sem->sleepers = 1;	/* us - see -1 above */
75		spin_unlock_irq(&semaphore_lock);
76
77		schedule();
78		tsk->state = TASK_UNINTERRUPTIBLE;
79		spin_lock_irq(&semaphore_lock);
80	}
81	spin_unlock_irq(&semaphore_lock);
82	remove_wait_queue(&sem->wait, &wait);
83	tsk->state = TASK_RUNNING;
84	wake_up(&sem->wait);
85}
86
87int __down_interruptible(struct semaphore * sem)
88{
89	int retval = 0;
90	struct task_struct *tsk = current;
91	DECLARE_WAITQUEUE(wait, tsk);
92	tsk->state = TASK_INTERRUPTIBLE;
93	add_wait_queue_exclusive(&sem->wait, &wait);
94
95	spin_lock_irq(&semaphore_lock);
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. The
117		 * "-1" is because we're still hoping to get
118		 * the lock.
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_irq(&semaphore_lock);
126
127		schedule();
128		tsk->state = TASK_INTERRUPTIBLE;
129		spin_lock_irq(&semaphore_lock);
130	}
131	spin_unlock_irq(&semaphore_lock);
132	tsk->state = TASK_RUNNING;
133	remove_wait_queue(&sem->wait, &wait);
134	wake_up(&sem->wait);
135	return retval;
136}
137
138/*
139 * Trylock failed - make sure we correct for
140 * having decremented the count.
141 */
142int __down_trylock(struct semaphore * sem)
143{
144        unsigned long flags;
145	int sleepers;
146
147	spin_lock_irqsave(&semaphore_lock, flags);
148	sleepers = sem->sleepers + 1;
149	sem->sleepers = 0;
150
151	/*
152	 * Add "everybody else" and us into it. They aren't
153	 * playing, because we own the spinlock.
154	 */
155	if (!atomic_add_negative(sleepers, &sem->count))
156		wake_up(&sem->wait);
157
158	spin_unlock_irqrestore(&semaphore_lock, flags);
159	return 1;
160}
161