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
__up(struct semaphore * sem)50 void __up(struct semaphore *sem)
51 {
52 wake_up(&sem->wait);
53 }
54
55 static spinlock_t semaphore_lock = SPIN_LOCK_UNLOCKED;
56
__down(struct semaphore * sem)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
__down_interruptible(struct semaphore * sem)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 */
__down_trylock(struct semaphore * sem)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 #if defined(CONFIG_FRAME_POINTER)
186 "pushl %ebp\n\t"
187 "movl %esp,%ebp\n\t"
188 #endif
189 "pushl %eax\n\t"
190 "pushl %edx\n\t"
191 "pushl %ecx\n\t"
192 "call __down\n\t"
193 "popl %ecx\n\t"
194 "popl %edx\n\t"
195 "popl %eax\n\t"
196 #if defined(CONFIG_FRAME_POINTER)
197 "movl %ebp,%esp\n\t"
198 "popl %ebp\n\t"
199 #endif
200 "ret"
201 );
202
203 asm(
204 ".text\n"
205 ".align 4\n"
206 ".globl __down_failed_interruptible\n"
207 "__down_failed_interruptible:\n\t"
208 #if defined(CONFIG_FRAME_POINTER)
209 "pushl %ebp\n\t"
210 "movl %esp,%ebp\n\t"
211 #endif
212 "pushl %edx\n\t"
213 "pushl %ecx\n\t"
214 "call __down_interruptible\n\t"
215 "popl %ecx\n\t"
216 "popl %edx\n\t"
217 #if defined(CONFIG_FRAME_POINTER)
218 "movl %ebp,%esp\n\t"
219 "popl %ebp\n\t"
220 #endif
221 "ret"
222 );
223
224 asm(
225 ".text\n"
226 ".align 4\n"
227 ".globl __down_failed_trylock\n"
228 "__down_failed_trylock:\n\t"
229 #if defined(CONFIG_FRAME_POINTER)
230 "pushl %ebp\n\t"
231 "movl %esp,%ebp\n\t"
232 #endif
233 "pushl %edx\n\t"
234 "pushl %ecx\n\t"
235 "call __down_trylock\n\t"
236 "popl %ecx\n\t"
237 "popl %edx\n\t"
238 #if defined(CONFIG_FRAME_POINTER)
239 "movl %ebp,%esp\n\t"
240 "popl %ebp\n\t"
241 #endif
242 "ret"
243 );
244
245 asm(
246 ".text\n"
247 ".align 4\n"
248 ".globl __up_wakeup\n"
249 "__up_wakeup:\n\t"
250 "pushl %eax\n\t"
251 "pushl %edx\n\t"
252 "pushl %ecx\n\t"
253 "call __up\n\t"
254 "popl %ecx\n\t"
255 "popl %edx\n\t"
256 "popl %eax\n\t"
257 "ret"
258 );
259
260 /*
261 * rw spinlock fallbacks
262 */
263 #if defined(CONFIG_SMP)
264 asm(
265 ".text\n"
266 ".align 4\n"
267 ".globl __write_lock_failed\n"
268 "__write_lock_failed:\n\t"
269 LOCK "addl $" RW_LOCK_BIAS_STR ",(%eax)\n"
270 "1: rep; nop\n\t"
271 "cmpl $" RW_LOCK_BIAS_STR ",(%eax)\n\t"
272 "jne 1b\n\t"
273 LOCK "subl $" RW_LOCK_BIAS_STR ",(%eax)\n\t"
274 "jnz __write_lock_failed\n\t"
275 "ret"
276 );
277
278 asm(
279 ".text\n"
280 ".align 4\n"
281 ".globl __read_lock_failed\n"
282 "__read_lock_failed:\n\t"
283 LOCK "incl (%eax)\n"
284 "1: rep; nop\n\t"
285 "cmpl $1,(%eax)\n\t"
286 "js 1b\n\t"
287 LOCK "decl (%eax)\n\t"
288 "js __read_lock_failed\n\t"
289 "ret"
290 );
291 #endif
292