Pull platform-drivers into test branch
[linux-2.6] / arch / arm26 / kernel / semaphore.c
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
53 void __up(struct semaphore *sem)
54 {
55         wake_up(&sem->wait);
56 }
57
58 static DEFINE_SPINLOCK(semaphore_lock);
59
60 void __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
93 int __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  */
152 int __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  */
182 asm("   .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
218 EXPORT_SYMBOL(__down_failed);
219 EXPORT_SYMBOL(__down_interruptible_failed);
220 EXPORT_SYMBOL(__down_trylock_failed);
221 EXPORT_SYMBOL(__up_wakeup);
222