Use private futexes for semaphores, unless they are initialized with pshared != 0.
Change-Id: I534e36a7171cd37037ae03b910ba71ea6968286d
Note: previously, sem_init() would return an error if pshared != 0.
diff --git a/libc/bionic/semaphore.c b/libc/bionic/semaphore.c
index b624943..39d049a 100644
--- a/libc/bionic/semaphore.c
+++ b/libc/bionic/semaphore.c
@@ -31,6 +31,16 @@
#include <sys/atomics.h>
#include <time.h>
#include <cutils/atomic-inline.h>
+#include <bionic_futex.h>
+
+/* Use the lower 31-bits for the counter, and the high bit for
+ * the shared flag.
+ */
+#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)
int sem_init(sem_t *sem, int pshared, unsigned int value)
{
@@ -39,12 +49,16 @@
return -1;
}
- if (pshared != 0) {
- errno = ENOSYS;
+ /* ensure that 'value' can be stored in the semaphore */
+ if ((value & SEM_VALUE_MASK) != value) {
+ errno = EINVAL;
return -1;
}
sem->count = value;
+ if (pshared != 0)
+ sem->count |= SEM_SHARED_MASK;
+
return 0;
}
@@ -55,10 +69,11 @@
errno = EINVAL;
return -1;
}
- if (sem->count == 0) {
+ if ((sem->count & SEM_VALUE_MASK) == 0) {
errno = EBUSY;
return -1;
}
+ sem->count = 0;
return 0;
}
@@ -91,32 +106,60 @@
}
+/* Return 0 if a semaphore's value is 0
+ * Otherwise, decrement the value and return the old value.
+ */
static int
-__atomic_dec_if_positive( volatile unsigned int* pvalue )
+__sem_dec_if_positive(volatile unsigned int *pvalue)
{
+ unsigned int shared = (*pvalue & SEM_SHARED_MASK);
unsigned int old;
do {
- old = *pvalue;
+ old = (*pvalue & SEM_VALUE_MASK);
}
- while ( old != 0 && __atomic_cmpxchg( (int)old, (int)old-1, (volatile int*)pvalue ) != 0 );
+ while ( old != 0 &&
+ __atomic_cmpxchg((int)(old|shared),
+ (int)((old-1)|shared),
+ (volatile int*)pvalue) != 0 );
return old;
}
+/* Increment the value of a semaphore atomically.
+ * NOTE: the value will wrap above SEM_VALUE_MASK
+ */
+static int
+__sem_inc(volatile unsigned int *pvalue)
+{
+ unsigned int shared = (*pvalue & SEM_SHARED_MASK);
+ unsigned int old;
+
+ 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;
+}
+
/* 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 (__atomic_dec_if_positive(&sem->count))
+ if (__sem_dec_if_positive(&sem->count))
break;
- __futex_wait(&sem->count, 0, 0);
+ __futex_wait_ex(&sem->count, shared, shared, NULL);
}
ANDROID_MEMBAR_FULL();
return 0;
@@ -125,6 +168,7 @@
int sem_timedwait(sem_t *sem, const struct timespec *abs_timeout)
{
int ret;
+ unsigned int shared;
if (sem == NULL) {
errno = EINVAL;
@@ -133,7 +177,7 @@
/* POSIX says we need to try to decrement the semaphore
* before checking the timeout value */
- if (__atomic_dec_if_positive(&sem->count)) {
+ if (__sem_dec_if_positive(&sem->count)) {
ANDROID_MEMBAR_FULL();
return 0;
}
@@ -148,6 +192,8 @@
return -1;
}
+ shared = SEM_GET_SHARED(sem);
+
for (;;) {
struct timespec ts;
int ret;
@@ -166,7 +212,7 @@
return -1;
}
- ret = __futex_wait(&sem->count, 0, &ts);
+ ret = __futex_wait_ex(&sem->count, shared, shared, &ts);
/* return in case of timeout or interrupt */
if (ret == -ETIMEDOUT || ret == -EINTR) {
@@ -174,7 +220,7 @@
return -1;
}
- if (__atomic_dec_if_positive(&sem->count)) {
+ if (__sem_dec_if_positive(&sem->count)) {
ANDROID_MEMBAR_FULL();
break;
}
@@ -185,12 +231,16 @@
/* unlock a semaphore */
int sem_post(sem_t *sem)
{
+ unsigned int shared;
+
if (sem == NULL)
return EINVAL;
+ shared = SEM_GET_SHARED(sem);
+
ANDROID_MEMBAR_FULL();
- if (__atomic_inc((volatile int*)&sem->count) >= 0)
- __futex_wake(&sem->count, 1);
+ if (__sem_inc(&sem->count) >= 0)
+ __futex_wake_ex(&sem->count, shared, 1);
return 0;
}
@@ -202,7 +252,7 @@
return -1;
}
- if (__atomic_dec_if_positive(&sem->count) > 0) {
+ if (__sem_dec_if_positive(&sem->count) > 0) {
ANDROID_MEMBAR_FULL();
return 0;
} else {
@@ -218,6 +268,6 @@
return -1;
}
- *sval = sem->count;
+ *sval = SEM_GET_VALUE(sem);
return 0;
}