1/*
2 *  ARM semaphore implementation, taken from
3 *
4 *  i386 semaphore implementation.
5 *
6 *  (C) Copyright 1999 Linus Torvalds
7 *  (C) Copyright 2003 Ian Molton (ARM26 mods)
8 *
9 *  Modified for ARM by Russell King
10 *
11 * This program is free software; you can redistribute it and/or modify
12 * it under the terms of the GNU General Public License version 2 as
13 * published by the Free Software Foundation.
14 */
15#include <linux/module.h>
16#include <linux/sched.h>
17#include <linux/errno.h>
18#include <linux/init.h>
19
20#include <asm/semaphore.h>
21
22/*
23 * Semaphores are implemented using a two-way counter:
24 * The "count" variable is decremented for each process
25 * that tries to acquire the semaphore, while the "sleeping"
26 * variable is a count of such acquires.
27 *
28 * Notably, the inline "up()" and "down()" functions can
29 * efficiently test if they need to do any extra work (up
30 * needs to do something only if count was negative before
31 * the increment operation.
32 *
33 * "sleeping" and the contention routine ordering is
34 * protected by the semaphore spinlock.
35 *
36 * Note that these functions are only called when there is
37 * contention on the lock, and as such all this is the
38 * "non-critical" part of the whole semaphore business. The
39 * critical part is the inline stuff in <asm/semaphore.h>
40 * where we want to avoid any extra jumps and calls.
41 */
42
43/*
44 * Logic:
45 *  - only on a boundary condition do we need to care. When we go
46 *    from a negative count to a non-negative, we wake people up.
47 *  - when we go from a non-negative count to a negative do we
48 *    (a) synchronize with the "sleeper" count and (b) make sure
49 *    that we're on the wakeup list before we synchronize so that
50 *    we cannot lose wakeup events.
51 */
52
53void __up(struct semaphore *sem)
54{
55	wake_up(&sem->wait);
56}
57
58static DEFINE_SPINLOCK(semaphore_lock);
59
60void __sched __down(struct semaphore * sem)
61{
62	struct task_struct *tsk = current;
63	DECLARE_WAITQUEUE(wait, tsk);
64	tsk->state = TASK_UNINTERRUPTIBLE;
65	add_wait_queue_exclusive(&sem->wait, &wait);
66
67	spin_lock_irq(&semaphore_lock);
68	sem->sleepers++;
69	for (;;) {
70		int sleepers = sem->sleepers;
71
72		/*
73		 * Add "everybody else" into it. They aren't
74		 * playing, because we own the spinlock.
75		 */
76		if (!atomic_add_negative(sleepers - 1, &sem->count)) {
77			sem->sleepers = 0;
78			break;
79		}
80		sem->sleepers = 1;	/* us - see -1 above */
81		spin_unlock_irq(&semaphore_lock);
82
83		schedule();
84		tsk->state = TASK_UNINTERRUPTIBLE;
85		spin_lock_irq(&semaphore_lock);
86	}
87	spin_unlock_irq(&semaphore_lock);
88	remove_wait_queue(&sem->wait, &wait);
89	tsk->state = TASK_RUNNING;
90	wake_up(&sem->wait);
91}
92
93int __sched __down_interruptible(struct semaphore * sem)
94{
95	int retval = 0;
96	struct task_struct *tsk = current;
97	DECLARE_WAITQUEUE(wait, tsk);
98	tsk->state = TASK_INTERRUPTIBLE;
99	add_wait_queue_exclusive(&sem->wait, &wait);
100
101	spin_lock_irq(&semaphore_lock);
102	sem->sleepers ++;
103	for (;;) {
104		int sleepers = sem->sleepers;
105
106		/*
107		 * With signals pending, this turns into
108		 * the trylock failure case - we won't be
109		 * sleeping, and we* can't get the lock as
110		 * it has contention. Just correct the count
111		 * and exit.
112		 */
113		if (signal_pending(current)) {
114			retval = -EINTR;
115			sem->sleepers = 0;
116			atomic_add(sleepers, &sem->count);
117			break;
118		}
119
120		/*
121		 * Add "everybody else" into it. They aren't
122		 * playing, because we own the spinlock. The
123		 * "-1" is because we're still hoping to get
124		 * the lock.
125		 */
126		if (!atomic_add_negative(sleepers - 1, &sem->count)) {
127			sem->sleepers = 0;
128			break;
129		}
130		sem->sleepers = 1;	/* us - see -1 above */
131		spin_unlock_irq(&semaphore_lock);
132
133		schedule();
134		tsk->state = TASK_INTERRUPTIBLE;
135		spin_lock_irq(&semaphore_lock);
136	}
137	spin_unlock_irq(&semaphore_lock);
138	tsk->state = TASK_RUNNING;
139	remove_wait_queue(&sem->wait, &wait);
140	wake_up(&sem->wait);
141	return retval;
142}
143
144/*
145 * Trylock failed - make sure we correct for
146 * having decremented the count.
147 *
148 * We could have done the trylock with a
149 * single "cmpxchg" without failure cases,
150 * but then it wouldn't work on a 386.
151 */
152int __down_trylock(struct semaphore * sem)
153{
154	int sleepers;
155	unsigned long flags;
156
157	spin_lock_irqsave(&semaphore_lock, flags);
158	sleepers = sem->sleepers + 1;
159	sem->sleepers = 0;
160
161	/*
162	 * Add "everybody else" and us into it. They aren't
163	 * playing, because we own the spinlock.
164	 */
165	if (!atomic_add_negative(sleepers, &sem->count))
166		wake_up(&sem->wait);
167
168	spin_unlock_irqrestore(&semaphore_lock, flags);
169	return 1;
170}
171
172/*
173 * The semaphore operations have a special calling sequence that
174 * allow us to do a simpler in-line version of them. These routines
175 * need to convert that sequence back into the C sequence when
176 * there is contention on the semaphore.
177 *
178 * ip contains the semaphore pointer on entry. Save the C-clobbered
179 * registers (r0 to r3 and lr), but not ip, as we use it as a return
180 * value in some cases..
181 */
182asm("	.section .sched.text , #alloc, #execinstr	\n\
183	.align	5				\n\
184	.globl	__down_failed			\n\
185__down_failed:					\n\
186	stmfd	sp!, {r0 - r3, lr}		\n\
187	mov	r0, ip				\n\
188	bl	__down				\n\
189	ldmfd	sp!, {r0 - r3, pc}^		\n\
190						\n\
191	.align	5				\n\
192	.globl	__down_interruptible_failed	\n\
193__down_interruptible_failed:			\n\
194	stmfd	sp!, {r0 - r3, lr}		\n\
195	mov	r0, ip				\n\
196	bl	__down_interruptible		\n\
197	mov	ip, r0				\n\
198	ldmfd	sp!, {r0 - r3, pc}^		\n\
199						\n\
200	.align	5				\n\
201	.globl	__down_trylock_failed		\n\
202__down_trylock_failed:				\n\
203	stmfd	sp!, {r0 - r3, lr}		\n\
204	mov	r0, ip				\n\
205	bl	__down_trylock			\n\
206	mov	ip, r0				\n\
207	ldmfd	sp!, {r0 - r3, pc}^		\n\
208						\n\
209	.align	5				\n\
210	.globl	__up_wakeup			\n\
211__up_wakeup:					\n\
212	stmfd	sp!, {r0 - r3, lr}		\n\
213	mov	r0, ip				\n\
214	bl	__up				\n\
215	ldmfd	sp!, {r0 - r3, pc}^		\n\
216	");
217
218EXPORT_SYMBOL(__down_failed);
219EXPORT_SYMBOL(__down_interruptible_failed);
220EXPORT_SYMBOL(__down_trylock_failed);
221EXPORT_SYMBOL(__up_wakeup);
222