#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
+ *
+ */
+
+/* Use the upper 31-bits for the counter, and the lower one
+ * for the shared flag.
*/
-#define SEM_VALUE_MASK 0x7fffffff
-#define SEM_SHARED_MASK 0x80000000
+#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)
-#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)
{
}
/* 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;
}
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;
}
}
-/* 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(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_dec_if_positive(volatile unsigned int *pvalue)
+__sem_trydec(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);
+ old = (*pvalue & SEMCOUNT_VALUE_MASK);
+ ret = SEMCOUNT_TO_VALUE(old);
+ if (ret <= 0)
+ break;
+
+ new = SEMCOUNT_DECREMENT(old);
}
- while ( old != 0 &&
- __atomic_cmpxchg((int)(old|shared),
- (int)((old-1)|shared),
- (volatile int*)pvalue) != 0 );
+ while (__atomic_cmpxchg((int)(old|shared),
+ (int)(new|shared),
+ (volatile int *)pvalue) != 0);
- return old;
+ return ret;
}
-/* Increment the value of a semaphore atomically.
- * NOTE: the value will wrap above SEM_VALUE_MASK
+
+/* "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 */
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;
}
/* 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 ||
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;
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;
}
return -1;
}
- if (__sem_dec_if_positive(&sem->count) > 0) {
+ if (__sem_trydec(&sem->count) > 0) {
ANDROID_MEMBAR_FULL();
return 0;
} else {
}
}
+/* 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;
}
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