semaphore.c revision 668da74ef1d4558f699374cb28a9df1a011db122 
1/*
2 * Copyright (C) 2008 The Android Open Source Project
3 * All rights reserved.
4 *
5 * Redistribution and use in source and binary forms, with or without
6 * modification, are permitted provided that the following conditions
7 * are met:
8 *  * Redistributions of source code must retain the above copyright
9 *    notice, this list of conditions and the following disclaimer.
10 *  * Redistributions in binary form must reproduce the above copyright
11 *    notice, this list of conditions and the following disclaimer in
12 *    the documentation and/or other materials provided with the
13 *    distribution.
14 *
15 * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
16 * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
17 * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS
18 * FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE
19 * COPYRIGHT OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT,
20 * INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING,
21 * BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS
22 * OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED
23 * AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
24 * OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT
25 * OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
26 * SUCH DAMAGE.
27 */
28#include <semaphore.h>
29#include <errno.h>
30#include <sys/time.h>
31#include <time.h>
32#include <limits.h>
33
34#include "private/bionic_atomic_inline.h"
35#include "private/bionic_futex.h"
36
37/* In this implementation, a semaphore contains a
38 * 31-bit signed value and a 1-bit 'shared' flag
39 * (for process-sharing purpose).
40 *
41 * We use the value -1 to indicate contention on the
42 * semaphore, 0 or more to indicate uncontended state,
43 * any value lower than -2 is invalid at runtime.
44 *
45 * State diagram:
46 *
47 * post(1)  ==> 2
48 * post(0)  ==> 1
49 * post(-1) ==> 1, then wake all waiters
50 *
51 * wait(2)  ==> 1
52 * wait(1)  ==> 0
53 * wait(0)  ==> -1 then wait for a wake up + loop
54 * wait(-1) ==> -1 then wait for a wake up + loop
55 *
56 */
57
58/* Use the upper 31-bits for the counter, and the lower one
59 * for the shared flag.
60 */
61#define SEMCOUNT_SHARED_MASK      0x00000001
62#define SEMCOUNT_VALUE_MASK       0xfffffffe
63#define SEMCOUNT_VALUE_SHIFT      1
64
65/* Maximum unsigned value that can be stored in the semaphore.
66 * One bit is used for the shared flag, another one for the
67 * sign bit, leaving us with only 30 bits.
68 */
69#define SEM_MAX_VALUE             0x3fffffff
70
71/* convert a value into the corresponding sem->count bit pattern */
72#define SEMCOUNT_FROM_VALUE(val)    (((val) << SEMCOUNT_VALUE_SHIFT) & SEMCOUNT_VALUE_MASK)
73
74/* convert a sem->count bit pattern into the corresponding signed value */
75#define SEMCOUNT_TO_VALUE(sval)  ((int)(sval) >> SEMCOUNT_VALUE_SHIFT)
76
77/* the value +1 as a sem->count bit-pattern. */
78#define SEMCOUNT_ONE              SEMCOUNT_FROM_VALUE(1)
79
80/* the value -1 as a sem->count bit-pattern. */
81#define SEMCOUNT_MINUS_ONE        SEMCOUNT_FROM_VALUE(-1)
82
83#define SEMCOUNT_DECREMENT(sval)    (((sval) - (1U << SEMCOUNT_VALUE_SHIFT)) & SEMCOUNT_VALUE_MASK)
84#define SEMCOUNT_INCREMENT(sval)    (((sval) + (1U << SEMCOUNT_VALUE_SHIFT)) & SEMCOUNT_VALUE_MASK)
85
86/* return the shared bitflag from a semaphore */
87#define SEM_GET_SHARED(sem)       ((sem)->count & SEMCOUNT_SHARED_MASK)
88
89
90int sem_init(sem_t *sem, int pshared, unsigned int value)
91{
92    if (sem == NULL) {
93        errno = EINVAL;
94        return -1;
95    }
96
97    /* ensure that 'value' can be stored in the semaphore */
98    if (value > SEM_MAX_VALUE) {
99        errno = EINVAL;
100        return -1;
101    }
102
103    sem->count = SEMCOUNT_FROM_VALUE(value);
104    if (pshared != 0)
105        sem->count |= SEMCOUNT_SHARED_MASK;
106
107    return 0;
108}
109
110
111int sem_destroy(sem_t *sem)
112{
113    int count;
114
115    if (sem == NULL) {
116        errno = EINVAL;
117        return -1;
118    }
119    count = SEMCOUNT_TO_VALUE(sem->count);
120    if (count < 0) {
121        errno = EBUSY;
122        return -1;
123    }
124    sem->count = 0;
125    return 0;
126}
127
128
129sem_t *sem_open(const char *name __unused, int oflag __unused, ...)
130{
131    errno = ENOSYS;
132    return SEM_FAILED;
133}
134
135
136int sem_close(sem_t *sem)
137{
138    if (sem == NULL) {
139        errno = EINVAL;
140        return -1;
141    }
142    errno = ENOSYS;
143    return -1;
144}
145
146
147int sem_unlink(const char* name __unused)
148{
149    errno = ENOSYS;
150    return -1;
151}
152
153
154/* Decrement a semaphore's value atomically,
155 * and return the old one. As a special case,
156 * this returns immediately if the value is
157 * negative (i.e. -1)
158 */
159static int
160__sem_dec(volatile unsigned int *pvalue)
161{
162    unsigned int shared = (*pvalue & SEMCOUNT_SHARED_MASK);
163    unsigned int old, new;
164    int          ret;
165
166    do {
167        old = (*pvalue & SEMCOUNT_VALUE_MASK);
168        ret = SEMCOUNT_TO_VALUE(old);
169        if (ret < 0)
170            break;
171
172        new = SEMCOUNT_DECREMENT(old);
173    }
174    while (__bionic_cmpxchg((int)(old|shared),
175                            (int)(new|shared),
176                            (volatile int *)pvalue) != 0);
177    return ret;
178}
179
180/* Same as __sem_dec, but will not touch anything if the
181 * value is already negative *or* 0. Returns the old value.
182 */
183static int
184__sem_trydec(volatile unsigned int *pvalue)
185{
186    unsigned int shared = (*pvalue & SEMCOUNT_SHARED_MASK);
187    unsigned int old, new;
188    int          ret;
189
190    do {
191        old = (*pvalue & SEMCOUNT_VALUE_MASK);
192        ret = SEMCOUNT_TO_VALUE(old);
193        if (ret <= 0)
194            break;
195
196        new = SEMCOUNT_DECREMENT(old);
197    }
198    while (__bionic_cmpxchg((int)(old|shared),
199                            (int)(new|shared),
200                            (volatile int *)pvalue) != 0);
201
202    return ret;
203}
204
205
206/* "Increment" the value of a semaphore atomically and
207 * return its old value. Note that this implements
208 * the special case of "incrementing" any negative
209 * value to +1 directly.
210 *
211 * NOTE: The value will _not_ wrap above SEM_VALUE_MAX
212 */
213static int
214__sem_inc(volatile unsigned int *pvalue)
215{
216    unsigned int  shared = (*pvalue & SEMCOUNT_SHARED_MASK);
217    unsigned int  old, new;
218    int           ret;
219
220    do {
221        old = (*pvalue & SEMCOUNT_VALUE_MASK);
222        ret = SEMCOUNT_TO_VALUE(old);
223
224        /* Can't go higher than SEM_MAX_VALUE */
225        if (ret == SEM_MAX_VALUE)
226            break;
227
228        /* If the counter is negative, go directly to +1,
229         * otherwise just increment */
230        if (ret < 0)
231            new = SEMCOUNT_ONE;
232        else
233            new = SEMCOUNT_INCREMENT(old);
234    }
235    while ( __bionic_cmpxchg((int)(old|shared),
236                             (int)(new|shared),
237                             (volatile int*)pvalue) != 0);
238
239    return ret;
240}
241
242/* lock a semaphore */
243int sem_wait(sem_t *sem)
244{
245    unsigned shared;
246
247    if (sem == NULL) {
248        errno = EINVAL;
249        return -1;
250    }
251
252    shared = SEM_GET_SHARED(sem);
253
254    for (;;) {
255        if (__sem_dec(&sem->count) > 0)
256            break;
257
258        __futex_wait_ex(&sem->count, shared, shared|SEMCOUNT_MINUS_ONE, NULL);
259    }
260    ANDROID_MEMBAR_FULL();
261    return 0;
262}
263
264int sem_timedwait(sem_t *sem, const struct timespec *abs_timeout)
265{
266    unsigned int shared;
267
268    if (sem == NULL) {
269        errno = EINVAL;
270        return -1;
271    }
272
273    /* POSIX says we need to try to decrement the semaphore
274     * before checking the timeout value. Note that if the
275     * value is currently 0, __sem_trydec() does nothing.
276     */
277    if (__sem_trydec(&sem->count) > 0) {
278        ANDROID_MEMBAR_FULL();
279        return 0;
280    }
281
282    /* Check it as per Posix */
283    if (abs_timeout == NULL    ||
284        abs_timeout->tv_sec < 0 ||
285        abs_timeout->tv_nsec < 0 ||
286        abs_timeout->tv_nsec >= 1000000000)
287    {
288        errno = EINVAL;
289        return -1;
290    }
291
292    shared = SEM_GET_SHARED(sem);
293
294    for (;;) {
295        struct timespec ts;
296        int             ret;
297
298        /* Posix mandates CLOCK_REALTIME here */
299        clock_gettime( CLOCK_REALTIME, &ts );
300        ts.tv_sec  = abs_timeout->tv_sec - ts.tv_sec;
301        ts.tv_nsec = abs_timeout->tv_nsec - ts.tv_nsec;
302        if (ts.tv_nsec < 0) {
303            ts.tv_nsec += 1000000000;
304            ts.tv_sec  -= 1;
305        }
306
307        if (ts.tv_sec < 0 || ts.tv_nsec < 0) {
308            errno = ETIMEDOUT;
309            return -1;
310        }
311
312        /* Try to grab the semaphore. If the value was 0, this
313         * will also change it to -1 */
314        if (__sem_dec(&sem->count) > 0) {
315            ANDROID_MEMBAR_FULL();
316            break;
317        }
318
319        /* Contention detected. wait for a wakeup event */
320        ret = __futex_wait_ex(&sem->count, shared, shared|SEMCOUNT_MINUS_ONE, &ts);
321
322        /* return in case of timeout or interrupt */
323        if (ret == -ETIMEDOUT || ret == -EINTR) {
324            errno = -ret;
325            return -1;
326        }
327    }
328    return 0;
329}
330
331/* Unlock a semaphore */
332int sem_post(sem_t *sem)
333{
334    unsigned int shared;
335    int          old;
336
337    if (sem == NULL)
338        return EINVAL;
339
340    shared = SEM_GET_SHARED(sem);
341
342    ANDROID_MEMBAR_FULL();
343    old = __sem_inc(&sem->count);
344    if (old < 0) {
345        /* contention on the semaphore, wake up all waiters */
346        __futex_wake_ex(&sem->count, shared, INT_MAX);
347    }
348    else if (old == SEM_MAX_VALUE) {
349        /* overflow detected */
350        errno = EOVERFLOW;
351        return -1;
352    }
353
354    return 0;
355}
356
357int  sem_trywait(sem_t *sem)
358{
359    if (sem == NULL) {
360        errno = EINVAL;
361        return -1;
362    }
363
364    if (__sem_trydec(&sem->count) > 0) {
365        ANDROID_MEMBAR_FULL();
366        return 0;
367    } else {
368        errno = EAGAIN;
369        return -1;
370    }
371}
372
373/* Note that Posix requires that sem_getvalue() returns, in
374 * case of contention, the negative of the number of waiting
375 * threads.
376 *
377 * However, code that depends on this negative value to be
378 * meaningful is most probably racy. The GLibc sem_getvalue()
379 * only returns the semaphore value, which is 0, in case of
380 * contention, so we will mimick this behaviour here instead
381 * for better compatibility.
382 */
383int  sem_getvalue(sem_t *sem, int *sval)
384{
385    int  val;
386
387    if (sem == NULL || sval == NULL) {
388        errno = EINVAL;
389        return -1;
390    }
391
392    val = SEMCOUNT_TO_VALUE(sem->count);
393    if (val < 0)
394        val = 0;
395
396    *sval = val;
397    return 0;
398}
399