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/arch/arm/kernel/semaphore.c

https://bitbucket.org/evzijst/gittest
C | 220 lines | 119 code | 20 blank | 81 comment | 6 complexity | 4a9f7c60fc7b7b12a873819089344106 MD5 | raw file
  1/*
  2 *  ARM semaphore implementation, taken from
  3 *
  4 *  i386 semaphore implementation.
  5 *
  6 *  (C) Copyright 1999 Linus Torvalds
  7 *
  8 *  Modified for ARM by Russell King
  9 *
 10 * This program is free software; you can redistribute it and/or modify
 11 * it under the terms of the GNU General Public License version 2 as
 12 * published by the Free Software Foundation.
 13 */
 14#include <linux/module.h>
 15#include <linux/sched.h>
 16#include <linux/errno.h>
 17#include <linux/init.h>
 18
 19#include <asm/semaphore.h>
 20
 21/*
 22 * Semaphores are implemented using a two-way counter:
 23 * The "count" variable is decremented for each process
 24 * that tries to acquire the semaphore, while the "sleeping"
 25 * variable is a count of such acquires.
 26 *
 27 * Notably, the inline "up()" and "down()" functions can
 28 * efficiently test if they need to do any extra work (up
 29 * needs to do something only if count was negative before
 30 * the increment operation.
 31 *
 32 * "sleeping" and the contention routine ordering is
 33 * protected by the semaphore spinlock.
 34 *
 35 * Note that these functions are only called when there is
 36 * contention on the lock, and as such all this is the
 37 * "non-critical" part of the whole semaphore business. The
 38 * critical part is the inline stuff in <asm/semaphore.h>
 39 * where we want to avoid any extra jumps and calls.
 40 */
 41
 42/*
 43 * Logic:
 44 *  - only on a boundary condition do we need to care. When we go
 45 *    from a negative count to a non-negative, we wake people up.
 46 *  - when we go from a non-negative count to a negative do we
 47 *    (a) synchronize with the "sleeper" count and (b) make sure
 48 *    that we're on the wakeup list before we synchronize so that
 49 *    we cannot lose wakeup events.
 50 */
 51
 52void __up(struct semaphore *sem)
 53{
 54	wake_up(&sem->wait);
 55}
 56
 57static DEFINE_SPINLOCK(semaphore_lock);
 58
 59void __sched __down(struct semaphore * sem)
 60{
 61	struct task_struct *tsk = current;
 62	DECLARE_WAITQUEUE(wait, tsk);
 63	tsk->state = TASK_UNINTERRUPTIBLE;
 64	add_wait_queue_exclusive(&sem->wait, &wait);
 65
 66	spin_lock_irq(&semaphore_lock);
 67	sem->sleepers++;
 68	for (;;) {
 69		int sleepers = sem->sleepers;
 70
 71		/*
 72		 * Add "everybody else" into it. They aren't
 73		 * playing, because we own the spinlock.
 74		 */
 75		if (!atomic_add_negative(sleepers - 1, &sem->count)) {
 76			sem->sleepers = 0;
 77			break;
 78		}
 79		sem->sleepers = 1;	/* us - see -1 above */
 80		spin_unlock_irq(&semaphore_lock);
 81
 82		schedule();
 83		tsk->state = TASK_UNINTERRUPTIBLE;
 84		spin_lock_irq(&semaphore_lock);
 85	}
 86	spin_unlock_irq(&semaphore_lock);
 87	remove_wait_queue(&sem->wait, &wait);
 88	tsk->state = TASK_RUNNING;
 89	wake_up(&sem->wait);
 90}
 91
 92int __sched __down_interruptible(struct semaphore * sem)
 93{
 94	int retval = 0;
 95	struct task_struct *tsk = current;
 96	DECLARE_WAITQUEUE(wait, tsk);
 97	tsk->state = TASK_INTERRUPTIBLE;
 98	add_wait_queue_exclusive(&sem->wait, &wait);
 99
100	spin_lock_irq(&semaphore_lock);
101	sem->sleepers ++;
102	for (;;) {
103		int sleepers = sem->sleepers;
104
105		/*
106		 * With signals pending, this turns into
107		 * the trylock failure case - we won't be
108		 * sleeping, and we* can't get the lock as
109		 * it has contention. Just correct the count
110		 * and exit.
111		 */
112		if (signal_pending(current)) {
113			retval = -EINTR;
114			sem->sleepers = 0;
115			atomic_add(sleepers, &sem->count);
116			break;
117		}
118
119		/*
120		 * Add "everybody else" into it. They aren't
121		 * playing, because we own the spinlock. The
122		 * "-1" is because we're still hoping to get
123		 * the lock.
124		 */
125		if (!atomic_add_negative(sleepers - 1, &sem->count)) {
126			sem->sleepers = 0;
127			break;
128		}
129		sem->sleepers = 1;	/* us - see -1 above */
130		spin_unlock_irq(&semaphore_lock);
131
132		schedule();
133		tsk->state = TASK_INTERRUPTIBLE;
134		spin_lock_irq(&semaphore_lock);
135	}
136	spin_unlock_irq(&semaphore_lock);
137	tsk->state = TASK_RUNNING;
138	remove_wait_queue(&sem->wait, &wait);
139	wake_up(&sem->wait);
140	return retval;
141}
142
143/*
144 * Trylock failed - make sure we correct for
145 * having decremented the count.
146 *
147 * We could have done the trylock with a
148 * single "cmpxchg" without failure cases,
149 * but then it wouldn't work on a 386.
150 */
151int __down_trylock(struct semaphore * sem)
152{
153	int sleepers;
154	unsigned long flags;
155
156	spin_lock_irqsave(&semaphore_lock, flags);
157	sleepers = sem->sleepers + 1;
158	sem->sleepers = 0;
159
160	/*
161	 * Add "everybody else" and us into it. They aren't
162	 * playing, because we own the spinlock.
163	 */
164	if (!atomic_add_negative(sleepers, &sem->count))
165		wake_up(&sem->wait);
166
167	spin_unlock_irqrestore(&semaphore_lock, flags);
168	return 1;
169}
170
171/*
172 * The semaphore operations have a special calling sequence that
173 * allow us to do a simpler in-line version of them. These routines
174 * need to convert that sequence back into the C sequence when
175 * there is contention on the semaphore.
176 *
177 * ip contains the semaphore pointer on entry. Save the C-clobbered
178 * registers (r0 to r3 and lr), but not ip, as we use it as a return
179 * value in some cases..
180 */
181asm("	.section .sched.text,\"ax\"		\n\
182	.align	5				\n\
183	.globl	__down_failed			\n\
184__down_failed:					\n\
185	stmfd	sp!, {r0 - r3, lr}		\n\
186	mov	r0, ip				\n\
187	bl	__down				\n\
188	ldmfd	sp!, {r0 - r3, pc}		\n\
189						\n\
190	.align	5				\n\
191	.globl	__down_interruptible_failed	\n\
192__down_interruptible_failed:			\n\
193	stmfd	sp!, {r0 - r3, lr}		\n\
194	mov	r0, ip				\n\
195	bl	__down_interruptible		\n\
196	mov	ip, r0				\n\
197	ldmfd	sp!, {r0 - r3, pc}		\n\
198						\n\
199	.align	5				\n\
200	.globl	__down_trylock_failed		\n\
201__down_trylock_failed:				\n\
202	stmfd	sp!, {r0 - r3, lr}		\n\
203	mov	r0, ip				\n\
204	bl	__down_trylock			\n\
205	mov	ip, r0				\n\
206	ldmfd	sp!, {r0 - r3, pc}		\n\
207						\n\
208	.align	5				\n\
209	.globl	__up_wakeup			\n\
210__up_wakeup:					\n\
211	stmfd	sp!, {r0 - r3, lr}		\n\
212	mov	r0, ip				\n\
213	bl	__up				\n\
214	ldmfd	sp!, {r0 - r3, pc}		\n\
215	");
216
217EXPORT_SYMBOL(__down_failed);
218EXPORT_SYMBOL(__down_interruptible_failed);
219EXPORT_SYMBOL(__down_trylock_failed);
220EXPORT_SYMBOL(__up_wakeup);