File: /usr/src/linux/arch/i386/kernel/semaphore.c
1 /*
2 * i386 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@redhat.com>
14 */
15 #include <linux/config.h>
16 #include <linux/sched.h>
17 #include <asm/semaphore.h>
18
19 /*
20 * Semaphores are implemented using a two-way counter:
21 * The "count" variable is decremented for each process
22 * that tries to acquire the semaphore, while the "sleeping"
23 * variable is a count of such acquires.
24 *
25 * Notably, the inline "up()" and "down()" functions can
26 * efficiently test if they need to do any extra work (up
27 * needs to do something only if count was negative before
28 * the increment operation.
29 *
30 * "sleeping" and the contention routine ordering is
31 * protected by the semaphore spinlock.
32 *
33 * Note that these functions are only called when there is
34 * contention on the lock, and as such all this is the
35 * "non-critical" part of the whole semaphore business. The
36 * critical part is the inline stuff in <asm/semaphore.h>
37 * where we want to avoid any extra jumps and calls.
38 */
39
40 /*
41 * Logic:
42 * - only on a boundary condition do we need to care. When we go
43 * from a negative count to a non-negative, we wake people up.
44 * - when we go from a non-negative count to a negative do we
45 * (a) synchronize with the "sleeper" count and (b) make sure
46 * that we're on the wakeup list before we synchronize so that
47 * we cannot lose wakeup events.
48 */
49
50 void __up(struct semaphore *sem)
51 {
52 wake_up(&sem->wait);
53 }
54
55 static spinlock_t semaphore_lock = SPIN_LOCK_UNLOCKED;
56
57 void __down(struct semaphore * sem)
58 {
59 struct task_struct *tsk = current;
60 DECLARE_WAITQUEUE(wait, tsk);
61 tsk->state = TASK_UNINTERRUPTIBLE;
62 add_wait_queue_exclusive(&sem->wait, &wait);
63
64 spin_lock_irq(&semaphore_lock);
65 sem->sleepers++;
66 for (;;) {
67 int sleepers = sem->sleepers;
68
69 /*
70 * Add "everybody else" into it. They aren't
71 * playing, because we own the spinlock.
72 */
73 if (!atomic_add_negative(sleepers - 1, &sem->count)) {
74 sem->sleepers = 0;
75 break;
76 }
77 sem->sleepers = 1; /* us - see -1 above */
78 spin_unlock_irq(&semaphore_lock);
79
80 schedule();
81 tsk->state = TASK_UNINTERRUPTIBLE;
82 spin_lock_irq(&semaphore_lock);
83 }
84 spin_unlock_irq(&semaphore_lock);
85 remove_wait_queue(&sem->wait, &wait);
86 tsk->state = TASK_RUNNING;
87 wake_up(&sem->wait);
88 }
89
90 int __down_interruptible(struct semaphore * sem)
91 {
92 int retval = 0;
93 struct task_struct *tsk = current;
94 DECLARE_WAITQUEUE(wait, tsk);
95 tsk->state = TASK_INTERRUPTIBLE;
96 add_wait_queue_exclusive(&sem->wait, &wait);
97
98 spin_lock_irq(&semaphore_lock);
99 sem->sleepers ++;
100 for (;;) {
101 int sleepers = sem->sleepers;
102
103 /*
104 * With signals pending, this turns into
105 * the trylock failure case - we won't be
106 * sleeping, and we* can't get the lock as
107 * it has contention. Just correct the count
108 * and exit.
109 */
110 if (signal_pending(current)) {
111 retval = -EINTR;
112 sem->sleepers = 0;
113 atomic_add(sleepers, &sem->count);
114 break;
115 }
116
117 /*
118 * Add "everybody else" into it. They aren't
119 * playing, because we own the spinlock. The
120 * "-1" is because we're still hoping to get
121 * the lock.
122 */
123 if (!atomic_add_negative(sleepers - 1, &sem->count)) {
124 sem->sleepers = 0;
125 break;
126 }
127 sem->sleepers = 1; /* us - see -1 above */
128 spin_unlock_irq(&semaphore_lock);
129
130 schedule();
131 tsk->state = TASK_INTERRUPTIBLE;
132 spin_lock_irq(&semaphore_lock);
133 }
134 spin_unlock_irq(&semaphore_lock);
135 tsk->state = TASK_RUNNING;
136 remove_wait_queue(&sem->wait, &wait);
137 wake_up(&sem->wait);
138 return retval;
139 }
140
141 /*
142 * Trylock failed - make sure we correct for
143 * having decremented the count.
144 *
145 * We could have done the trylock with a
146 * single "cmpxchg" without failure cases,
147 * but then it wouldn't work on a 386.
148 */
149 int __down_trylock(struct semaphore * sem)
150 {
151 int sleepers;
152 unsigned long flags;
153
154 spin_lock_irqsave(&semaphore_lock, flags);
155 sleepers = sem->sleepers + 1;
156 sem->sleepers = 0;
157
158 /*
159 * Add "everybody else" and us into it. They aren't
160 * playing, because we own the spinlock.
161 */
162 if (!atomic_add_negative(sleepers, &sem->count))
163 wake_up(&sem->wait);
164
165 spin_unlock_irqrestore(&semaphore_lock, flags);
166 return 1;
167 }
168
169
170 /*
171 * The semaphore operations have a special calling sequence that
172 * allow us to do a simpler in-line version of them. These routines
173 * need to convert that sequence back into the C sequence when
174 * there is contention on the semaphore.
175 *
176 * %ecx contains the semaphore pointer on entry. Save the C-clobbered
177 * registers (%eax, %edx and %ecx) except %eax when used as a return
178 * value..
179 */
180 asm(
181 ".text\n"
182 ".align 4\n"
183 ".globl __down_failed\n"
184 "__down_failed:\n\t"
185 "pushl %eax\n\t"
186 "pushl %edx\n\t"
187 "pushl %ecx\n\t"
188 "call __down\n\t"
189 "popl %ecx\n\t"
190 "popl %edx\n\t"
191 "popl %eax\n\t"
192 "ret"
193 );
194
195 asm(
196 ".text\n"
197 ".align 4\n"
198 ".globl __down_failed_interruptible\n"
199 "__down_failed_interruptible:\n\t"
200 "pushl %edx\n\t"
201 "pushl %ecx\n\t"
202 "call __down_interruptible\n\t"
203 "popl %ecx\n\t"
204 "popl %edx\n\t"
205 "ret"
206 );
207
208 asm(
209 ".text\n"
210 ".align 4\n"
211 ".globl __down_failed_trylock\n"
212 "__down_failed_trylock:\n\t"
213 "pushl %edx\n\t"
214 "pushl %ecx\n\t"
215 "call __down_trylock\n\t"
216 "popl %ecx\n\t"
217 "popl %edx\n\t"
218 "ret"
219 );
220
221 asm(
222 ".text\n"
223 ".align 4\n"
224 ".globl __up_wakeup\n"
225 "__up_wakeup:\n\t"
226 "pushl %eax\n\t"
227 "pushl %edx\n\t"
228 "pushl %ecx\n\t"
229 "call __up\n\t"
230 "popl %ecx\n\t"
231 "popl %edx\n\t"
232 "popl %eax\n\t"
233 "ret"
234 );
235
236 /*
237 * rw spinlock fallbacks
238 */
239 #if defined(CONFIG_SMP)
240 asm(
241 "
242 .align 4
243 .globl __write_lock_failed
244 __write_lock_failed:
245 " LOCK "addl $" RW_LOCK_BIAS_STR ",(%eax)
246 1: cmpl $" RW_LOCK_BIAS_STR ",(%eax)
247 jne 1b
248
249 " LOCK "subl $" RW_LOCK_BIAS_STR ",(%eax)
250 jnz __write_lock_failed
251 ret
252
253
254 .align 4
255 .globl __read_lock_failed
256 __read_lock_failed:
257 lock ; incl (%eax)
258 1: cmpl $1,(%eax)
259 js 1b
260
261 lock ; decl (%eax)
262 js __read_lock_failed
263 ret
264 "
265 );
266 #endif
267