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libc: Fix sem_post() implementation to wake up all waiting threads.

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This also allows us to optimize the case where we increment an
uncontended semaphore (no need to call futex_wake() then).

Change-Id: Iad48efe8551dc66dc89d3e3f18c001e5a6c1939f
This commit is contained in:
David 'Digit' Turner 2010-06-28 14:10:14 -07:00
parent a02b93bd75
commit 519763265e
3 changed files with 181 additions and 49 deletions
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223
libc/bionic/semaphore.c
View file

@ -32,15 +32,60 @@
#include <time.h>
#include <bionic_atomic_inline.h>
#include <bionic_futex.h>
#include <limits.h>
/* Use the lower 31-bits for the counter, and the high bit for
* the shared flag.
/* 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
*
*/
#define SEM_VALUE_MASK 0x7fffffff
#define SEM_SHARED_MASK 0x80000000
#define SEM_GET_SHARED(sem) ((sem)->count & SEM_SHARED_MASK)
#define SEM_GET_VALUE(sem) ((sem)->count & SEM_VALUE_MASK)
/* 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)
{
@ -50,14 +95,14 @@ int sem_init(sem_t *sem, int pshared, unsigned int value)
}
/* ensure that 'value' can be stored in the semaphore */
if ((value & SEM_VALUE_MASK) != value) {
if (value > SEM_MAX_VALUE) {
errno = EINVAL;
return -1;
}
sem->count = value;
sem->count = SEMCOUNT_FROM_VALUE(value);
if (pshared != 0)
sem->count |= SEM_SHARED_MASK;
sem->count |= SEMCOUNT_SHARED_MASK;
return 0;
}
@ -65,11 +110,14 @@ int sem_init(sem_t *sem, int pshared, unsigned int value)
int sem_destroy(sem_t *sem)
{
int count;
if (sem == NULL) {
errno = EINVAL;
return -1;
}
if ((sem->count & SEM_VALUE_MASK) == 0) {
count = SEMCOUNT_TO_VALUE(sem->count);
if (count < 0) {
errno = EBUSY;
return -1;
}
@ -106,41 +154,92 @@ int sem_unlink(const char * name)
}
/* Return 0 if a semaphore's value is 0
* Otherwise, decrement the value and return the old value.
/* 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_if_positive(volatile unsigned int *pvalue)
__sem_dec(volatile unsigned int *pvalue)
{
unsigned int shared = (*pvalue & SEM_SHARED_MASK);
unsigned int old;
unsigned int shared = (*pvalue & SEMCOUNT_SHARED_MASK);
unsigned int old, new;
int ret;
do {
old = (*pvalue & SEM_VALUE_MASK);
}
while ( old != 0 &&
__atomic_cmpxchg((int)(old|shared),
(int)((old-1)|shared),
(volatile int*)pvalue) != 0 );
old = (*pvalue & SEMCOUNT_VALUE_MASK);
ret = SEMCOUNT_TO_VALUE(old);
if (ret < 0)
break;
return old;
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.
* NOTE: the value will wrap above SEM_VALUE_MASK
/* 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 & SEM_SHARED_MASK);
unsigned int old;
unsigned int shared = (*pvalue & SEMCOUNT_SHARED_MASK);
unsigned int old, new;
int ret;
do {
old = (*pvalue & SEM_VALUE_MASK);
} while ( __atomic_cmpxchg((int)(old|shared),
(int)(((old+1)&SEM_VALUE_MASK)|shared),
(volatile int*)pvalue) != 0);
return old;
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 */
@ -156,10 +255,10 @@ int sem_wait(sem_t *sem)
shared = SEM_GET_SHARED(sem);
for (;;) {
if (__sem_dec_if_positive(&sem->count))
if (__sem_dec(&sem->count) > 0)
break;
__futex_wait_ex(&sem->count, shared, shared, NULL);
__futex_wait_ex(&sem->count, shared, shared|SEMCOUNT_MINUS_ONE, NULL);
}
ANDROID_MEMBAR_FULL();
return 0;
@ -176,13 +275,15 @@ int sem_timedwait(sem_t *sem, const struct timespec *abs_timeout)
}
/* POSIX says we need to try to decrement the semaphore
* before checking the timeout value */
if (__sem_dec_if_positive(&sem->count)) {
* 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 */
/* Check it as per Posix */
if (abs_timeout == NULL ||
abs_timeout->tv_sec < 0 ||
abs_timeout->tv_nsec < 0 ||
@ -212,26 +313,30 @@ int sem_timedwait(sem_t *sem, const struct timespec *abs_timeout)
return -1;
}
ret = __futex_wait_ex(&sem->count, shared, shared, &ts);
/* 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;
}
if (__sem_dec_if_positive(&sem->count)) {
ANDROID_MEMBAR_FULL();
break;
}
}
return 0;
}
/* unlock a semaphore */
/* Unlock a semaphore */
int sem_post(sem_t *sem)
{
unsigned int shared;
int old;
if (sem == NULL)
return EINVAL;
@ -239,8 +344,16 @@ int sem_post(sem_t *sem)
shared = SEM_GET_SHARED(sem);
ANDROID_MEMBAR_FULL();
if (__sem_inc(&sem->count) >= 0)
__futex_wake_ex(&sem->count, shared, 1);
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;
}
@ -252,7 +365,7 @@ int sem_trywait(sem_t *sem)
return -1;
}
if (__sem_dec_if_positive(&sem->count) > 0) {
if (__sem_trydec(&sem->count) > 0) {
ANDROID_MEMBAR_FULL();
return 0;
} else {
@ -261,13 +374,29 @@ int sem_trywait(sem_t *sem)
}
}
/* 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;
}
*sval = SEM_GET_VALUE(sem);
val = SEMCOUNT_TO_VALUE(sem->count);
if (val < 0)
val = 0;
*sval = val;
return 0;
}

5
libc/docs/CHANGES.TXT
View file

@ -10,6 +10,10 @@ Differences between current and Android 2.2:
- <semaphore.h>: Use private futexes for semaphore implementation,
unless your set 'pshared' to non-0 when calling sem_init().
Also fixed a bug in sem_post() to make it wake up all waiting
threads, instead of one. As a consequence, the maximum semaphore
value is now reduced to 0x3fffffff.
- <math.h>: Added sincos(), sincosf() and sincosl() (GLibc compatibility).
- <sys/sysinfo.h>: Added missing sysinfo() system call implementation
@ -59,7 +63,6 @@ Differences between current and Android 2.2:
- <sys/select.h>: add missing declaration for pselect()
-
-------------------------------------------------------------------------------
Differences between Android 2.2. and Android 2.1:

2
libc/unistd/sysconf.c
View file

@ -44,7 +44,7 @@
#define SYSTEM_MQ_OPEN_MAX 8
#define SYSTEM_MQ_PRIO_MAX 32768
#define SYSTEM_SEM_NSEMS_MAX 256
#define SYSTEM_SEM_VALUE_MAX (2147483647)
#define SYSTEM_SEM_VALUE_MAX 0x3fffffff /* see bionic/semaphore.c */
#define SYSTEM_SIGQUEUE_MAX 32
#define SYSTEM_TIMER_MAX 32
#define SYSTEM_LOGIN_NAME_MAX 256