/*
* Copyright (C) 2008 The Android Open Source Project
* All rights reserved.
*
* Redistribution and use in source and binary forms, with or without
* modification, are permitted provided that the following conditions
* are met:
* * Redistributions of source code must retain the above copyright
* notice, this list of conditions and the following disclaimer.
* * Redistributions in binary form must reproduce the above copyright
* notice, this list of conditions and the following disclaimer in
* the documentation and/or other materials provided with the
* distribution.
*
* THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
* "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
* LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS
* FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE
* COPYRIGHT OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT,
* INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING,
* BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS
* OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED
* AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
* OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT
* OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
* SUCH DAMAGE.
*/
#include <semaphore.h>
#include <errno.h>
#include <sys/time.h>
#include <sys/atomics.h>
#include <time.h>
#include <bionic_atomic_inline.h>
#include <bionic_futex.h>
#include <limits.h>
/* In this implementation, a semaphore contains a
* 31-bit signed value and a 1-bit 'shared' flag
* (for process-sharing purpose).
*
* We use the value -1 to indicate contention on the
* semaphore, 0 or more to indicate uncontended state,
* any value lower than -2 is invalid at runtime.
*
* State diagram:
*
* post(1) ==> 2
* post(0) ==> 1
* post(-1) ==> 1, then wake all waiters
*
* wait(2) ==> 1
* wait(1) ==> 0
* wait(0) ==> -1 then wait for a wake up + loop
* wait(-1) ==> -1 then wait for a wake up + loop
*
*/
/* Use the upper 31-bits for the counter, and the lower one
* for the shared flag.
*/
#define SEMCOUNT_SHARED_MASK 0x00000001
#define SEMCOUNT_VALUE_MASK 0xfffffffe
#define SEMCOUNT_VALUE_SHIFT 1
/* Maximum unsigned value that can be stored in the semaphore.
* One bit is used for the shared flag, another one for the
* sign bit, leaving us with only 30 bits.
*/
#define SEM_MAX_VALUE 0x3fffffff
/* convert a value into the corresponding sem->count bit pattern */
#define SEMCOUNT_FROM_VALUE(val) (((val) << SEMCOUNT_VALUE_SHIFT) & SEMCOUNT_VALUE_MASK)
/* convert a sem->count bit pattern into the corresponding signed value */
#define SEMCOUNT_TO_VALUE(sval) ((int)(sval) >> SEMCOUNT_VALUE_SHIFT)
/* the value +1 as a sem->count bit-pattern. */
#define SEMCOUNT_ONE SEMCOUNT_FROM_VALUE(1)
/* the value -1 as a sem->count bit-pattern. */
#define SEMCOUNT_MINUS_ONE SEMCOUNT_FROM_VALUE(-1)
#define SEMCOUNT_DECREMENT(sval) (((sval) - (1U << SEMCOUNT_VALUE_SHIFT)) & SEMCOUNT_VALUE_MASK)
#define SEMCOUNT_INCREMENT(sval) (((sval) + (1U << SEMCOUNT_VALUE_SHIFT)) & SEMCOUNT_VALUE_MASK)
/* return the shared bitflag from a semaphore */
#define SEM_GET_SHARED(sem) ((sem)->count & SEMCOUNT_SHARED_MASK)
int sem_init(sem_t *sem, int pshared, unsigned int value)
{
if (sem == NULL) {
errno = EINVAL;
return -1;
}
/* ensure that 'value' can be stored in the semaphore */
if (value > SEM_MAX_VALUE) {
errno = EINVAL;
return -1;
}
sem->count = SEMCOUNT_FROM_VALUE(value);
if (pshared != 0)
sem->count |= SEMCOUNT_SHARED_MASK;
return 0;
}
int sem_destroy(sem_t *sem)
{
int count;
if (sem == NULL) {
errno = EINVAL;
return -1;
}
count = SEMCOUNT_TO_VALUE(sem->count);
if (count < 0) {
errno = EBUSY;
return -1;
}
sem->count = 0;
return 0;
}
sem_t *sem_open(const char *name, int oflag, ...)
{
name=name;
oflag=oflag;
errno = ENOSYS;
return SEM_FAILED;
}
int sem_close(sem_t *sem)
{
if (sem == NULL) {
errno = EINVAL;
return -1;
}
errno = ENOSYS;
return -1;
}
int sem_unlink(const char * name)
{
errno = ENOSYS;
return -1;
}
/* Decrement a semaphore's value atomically,
* and return the old one. As a special case,
* this returns immediately if the value is
* negative (i.e. -1)
*/
static int
__sem_dec(volatile unsigned int *pvalue)
{
unsigned int shared = (*pvalue & SEMCOUNT_SHARED_MASK);
unsigned int old, new;
int ret;
do {
old = (*pvalue & SEMCOUNT_VALUE_MASK);
ret = SEMCOUNT_TO_VALUE(old);
if (ret < 0)
break;
new = SEMCOUNT_DECREMENT(old);
}
while (__atomic_cmpxchg((int)(old|shared),
(int)(new|shared),
(volatile int *)pvalue) != 0);
return ret;
}
/* Same as __sem_dec, but will not touch anything if the
* value is already negative *or* 0. Returns the old value.
*/
static int
__sem_trydec(volatile unsigned int *pvalue)
{
unsigned int shared = (*pvalue & SEMCOUNT_SHARED_MASK);
unsigned int old, new;
int ret;
do {
old = (*pvalue & SEMCOUNT_VALUE_MASK);
ret = SEMCOUNT_TO_VALUE(old);
if (ret <= 0)
break;
new = SEMCOUNT_DECREMENT(old);
}
while (__atomic_cmpxchg((int)(old|shared),
(int)(new|shared),
(volatile int *)pvalue) != 0);
return ret;
}
/* "Increment" the value of a semaphore atomically and
* return its old value. Note that this implements
* the special case of "incrementing" any negative
* value to +1 directly.
*
* NOTE: The value will _not_ wrap above SEM_VALUE_MAX
*/
static int
__sem_inc(volatile unsigned int *pvalue)
{
unsigned int shared = (*pvalue & SEMCOUNT_SHARED_MASK);
unsigned int old, new;
int ret;
do {
old = (*pvalue & SEMCOUNT_VALUE_MASK);
ret = SEMCOUNT_TO_VALUE(old);
/* Can't go higher than SEM_MAX_VALUE */
if (ret == SEM_MAX_VALUE)
break;
/* If the counter is negative, go directly to +1,
* otherwise just increment */
if (ret < 0)
new = SEMCOUNT_ONE;
else
new = SEMCOUNT_INCREMENT(old);
}
while ( __atomic_cmpxchg((int)(old|shared),
(int)(new|shared),
(volatile int*)pvalue) != 0);
return ret;
}
/* lock a semaphore */
int sem_wait(sem_t *sem)
{
unsigned shared;
if (sem == NULL) {
errno = EINVAL;
return -1;
}
shared = SEM_GET_SHARED(sem);
for (;;) {
if (__sem_dec(&sem->count) > 0)
break;
__futex_wait_ex(&sem->count, shared, shared|SEMCOUNT_MINUS_ONE, NULL);
}
ANDROID_MEMBAR_FULL();
return 0;
}
int sem_timedwait(sem_t *sem, const struct timespec *abs_timeout)
{
int ret;
unsigned int shared;
if (sem == NULL) {
errno = EINVAL;
return -1;
}
/* POSIX says we need to try to decrement the semaphore
* before checking the timeout value. Note that if the
* value is currently 0, __sem_trydec() does nothing.
*/
if (__sem_trydec(&sem->count) > 0) {
ANDROID_MEMBAR_FULL();
return 0;
}
/* Check it as per Posix */
if (abs_timeout == NULL ||
abs_timeout->tv_sec < 0 ||
abs_timeout->tv_nsec < 0 ||
abs_timeout->tv_nsec >= 1000000000)
{
errno = EINVAL;
return -1;
}
shared = SEM_GET_SHARED(sem);
for (;;) {
struct timespec ts;
int ret;
/* Posix mandates CLOCK_REALTIME here */
clock_gettime( CLOCK_REALTIME, &ts );
ts.tv_sec = abs_timeout->tv_sec - ts.tv_sec;
ts.tv_nsec = abs_timeout->tv_nsec - ts.tv_nsec;
if (ts.tv_nsec < 0) {
ts.tv_nsec += 1000000000;
ts.tv_sec -= 1;
}
if (ts.tv_sec < 0 || ts.tv_nsec < 0) {
errno = ETIMEDOUT;
return -1;
}
/* Try to grab the semaphore. If the value was 0, this
* will also change it to -1 */
if (__sem_dec(&sem->count) > 0) {
ANDROID_MEMBAR_FULL();
break;
}
/* Contention detected. wait for a wakeup event */
ret = __futex_wait_ex(&sem->count, shared, shared|SEMCOUNT_MINUS_ONE, &ts);
/* return in case of timeout or interrupt */
if (ret == -ETIMEDOUT || ret == -EINTR) {
errno = -ret;
return -1;
}
}
return 0;
}
/* Unlock a semaphore */
int sem_post(sem_t *sem)
{
unsigned int shared;
int old;
if (sem == NULL)
return EINVAL;
shared = SEM_GET_SHARED(sem);
ANDROID_MEMBAR_FULL();
old = __sem_inc(&sem->count);
if (old < 0) {
/* contention on the semaphore, wake up all waiters */
__futex_wake_ex(&sem->count, shared, INT_MAX);
}
else if (old == SEM_MAX_VALUE) {
/* overflow detected */
errno = EOVERFLOW;
return -1;
}
return 0;
}
int sem_trywait(sem_t *sem)
{
if (sem == NULL) {
errno = EINVAL;
return -1;
}
if (__sem_trydec(&sem->count) > 0) {
ANDROID_MEMBAR_FULL();
return 0;
} else {
errno = EAGAIN;
return -1;
}
}
/* Note that Posix requires that sem_getvalue() returns, in
* case of contention, the negative of the number of waiting
* threads.
*
* However, code that depends on this negative value to be
* meaningful is most probably racy. The GLibc sem_getvalue()
* only returns the semaphore value, which is 0, in case of
* contention, so we will mimick this behaviour here instead
* for better compatibility.
*/
int sem_getvalue(sem_t *sem, int *sval)
{
int val;
if (sem == NULL || sval == NULL) {
errno = EINVAL;
return -1;
}
val = SEMCOUNT_TO_VALUE(sem->count);
if (val < 0)
val = 0;
*sval = val;
return 0;
}