[PATCH] CONFIG_IA32
[linux-2.6.git] / lib / semaphore-sleepers.c
1 /*
2  * i386 and x86-64 semaphore implementation.
3  *
4  * (C) Copyright 1999 Linus Torvalds
5  *
6  * Portions Copyright 1999 Red Hat, Inc.
7  *
8  *      This program is free software; you can redistribute it and/or
9  *      modify it under the terms of the GNU General Public License
10  *      as published by the Free Software Foundation; either version
11  *      2 of the License, or (at your option) any later version.
12  *
13  * rw semaphores implemented November 1999 by Benjamin LaHaise <bcrl@kvack.org>
14  */
15 #include <linux/config.h>
16 #include <linux/sched.h>
17 #include <linux/err.h>
18 #include <linux/init.h>
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 protected
33  * by the spinlock in the semaphore's waitqueue head.
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
52 fastcall void __up(struct semaphore *sem)
53 {
54         wake_up(&sem->wait);
55 }
56
57 fastcall void __sched __down(struct semaphore * sem)
58 {
59         struct task_struct *tsk = current;
60         DECLARE_WAITQUEUE(wait, tsk);
61         unsigned long flags;
62
63         tsk->state = TASK_UNINTERRUPTIBLE;
64         spin_lock_irqsave(&sem->wait.lock, flags);
65         add_wait_queue_exclusive_locked(&sem->wait, &wait);
66
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 in
74                  * the wait_queue_head.
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_irqrestore(&sem->wait.lock, flags);
82
83                 schedule();
84
85                 spin_lock_irqsave(&sem->wait.lock, flags);
86                 tsk->state = TASK_UNINTERRUPTIBLE;
87         }
88         remove_wait_queue_locked(&sem->wait, &wait);
89         wake_up_locked(&sem->wait);
90         spin_unlock_irqrestore(&sem->wait.lock, flags);
91         tsk->state = TASK_RUNNING;
92 }
93
94 fastcall int __sched __down_interruptible(struct semaphore * sem)
95 {
96         int retval = 0;
97         struct task_struct *tsk = current;
98         DECLARE_WAITQUEUE(wait, tsk);
99         unsigned long flags;
100
101         tsk->state = TASK_INTERRUPTIBLE;
102         spin_lock_irqsave(&sem->wait.lock, flags);
103         add_wait_queue_exclusive_locked(&sem->wait, &wait);
104
105         sem->sleepers++;
106         for (;;) {
107                 int sleepers = sem->sleepers;
108
109                 /*
110                  * With signals pending, this turns into
111                  * the trylock failure case - we won't be
112                  * sleeping, and we* can't get the lock as
113                  * it has contention. Just correct the count
114                  * and exit.
115                  */
116                 if (signal_pending(current)) {
117                         retval = -EINTR;
118                         sem->sleepers = 0;
119                         atomic_add(sleepers, &sem->count);
120                         break;
121                 }
122
123                 /*
124                  * Add "everybody else" into it. They aren't
125                  * playing, because we own the spinlock in
126                  * wait_queue_head. The "-1" is because we're
127                  * still hoping to get the semaphore.
128                  */
129                 if (!atomic_add_negative(sleepers - 1, &sem->count)) {
130                         sem->sleepers = 0;
131                         break;
132                 }
133                 sem->sleepers = 1;      /* us - see -1 above */
134                 spin_unlock_irqrestore(&sem->wait.lock, flags);
135
136                 schedule();
137
138                 spin_lock_irqsave(&sem->wait.lock, flags);
139                 tsk->state = TASK_INTERRUPTIBLE;
140         }
141         remove_wait_queue_locked(&sem->wait, &wait);
142         wake_up_locked(&sem->wait);
143         spin_unlock_irqrestore(&sem->wait.lock, flags);
144
145         tsk->state = TASK_RUNNING;
146         return retval;
147 }
148
149 /*
150  * Trylock failed - make sure we correct for
151  * having decremented the count.
152  *
153  * We could have done the trylock with a
154  * single "cmpxchg" without failure cases,
155  * but then it wouldn't work on a 386.
156  */
157 fastcall int __down_trylock(struct semaphore * sem)
158 {
159         int sleepers;
160         unsigned long flags;
161
162         spin_lock_irqsave(&sem->wait.lock, flags);
163         sleepers = sem->sleepers + 1;
164         sem->sleepers = 0;
165
166         /*
167          * Add "everybody else" and us into it. They aren't
168          * playing, because we own the spinlock in the
169          * wait_queue_head.
170          */
171         if (!atomic_add_negative(sleepers, &sem->count)) {
172                 wake_up_locked(&sem->wait);
173         }
174
175         spin_unlock_irqrestore(&sem->wait.lock, flags);
176         return 1;
177 }