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Switch sem_t from bionic atomics to stdatomic.h.

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Bug: 17572887
Change-Id: If66851ba9b831cdd698b9f1303289bb14448bd03
This commit is contained in:
Yabin Cui 2015-01-30 12:30:08 -08:00
parent 7f17aea2fc
commit a3d97da4c5
2 changed files with 86 additions and 65 deletions
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149
libc/bionic/semaphore.cpp
View File

@ -26,13 +26,19 @@
* SUCH DAMAGE.
*/
// Memory order requirements for POSIX semaphores appear unclear and are
// currently interpreted inconsistently.
// We conservatively prefer sequentially consistent operations for now.
// CAUTION: This is more conservative than some other major implementations,
// and may change if and when the issue is resolved.
#include <semaphore.h>
#include <errno.h>
#include <limits.h>
#include <stdatomic.h>
#include <sys/time.h>
#include <time.h>
#include "private/bionic_atomic_inline.h"
#include "private/bionic_constants.h"
#include "private/bionic_futex.h"
#include "private/bionic_time_conversions.h"
@ -66,7 +72,7 @@
#define SEMCOUNT_FROM_VALUE(val) (((val) << SEMCOUNT_VALUE_SHIFT) & SEMCOUNT_VALUE_MASK)
// Convert a sem->count bit pattern into the corresponding signed value.
static inline int SEMCOUNT_TO_VALUE(uint32_t sval) {
static inline int SEMCOUNT_TO_VALUE(unsigned int sval) {
return (static_cast<int>(sval) >> SEMCOUNT_VALUE_SHIFT);
}
@ -79,11 +85,20 @@ static inline int SEMCOUNT_TO_VALUE(uint32_t sval) {
#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.
static inline uint32_t SEM_GET_SHARED(sem_t* sem) {
return (sem->count & SEMCOUNT_SHARED_MASK);
static inline atomic_uint* SEM_TO_ATOMIC_POINTER(sem_t* sem) {
static_assert(sizeof(atomic_uint) == sizeof(sem->count),
"sem->count should actually be atomic_uint in implementation.");
// We prefer casting to atomic_uint instead of declaring sem->count to be atomic_uint directly.
// Because using the second method pollutes semaphore.h.
return reinterpret_cast<atomic_uint*>(&sem->count);
}
// Return the shared bitflag from a semaphore counter.
static inline unsigned int SEM_GET_SHARED(atomic_uint* sem_count_ptr) {
// memory_order_relaxed is used as SHARED flag will not be changed after init.
return (atomic_load_explicit(sem_count_ptr, memory_order_relaxed) & SEMCOUNT_SHARED_MASK);
}
int sem_init(sem_t* sem, int pshared, unsigned int value) {
// Ensure that 'value' can be stored in the semaphore.
@ -92,10 +107,13 @@ int sem_init(sem_t* sem, int pshared, unsigned int value) {
return -1;
}
sem->count = SEMCOUNT_FROM_VALUE(value);
unsigned int count = SEMCOUNT_FROM_VALUE(value);
if (pshared != 0) {
sem->count |= SEMCOUNT_SHARED_MASK;
count |= SEMCOUNT_SHARED_MASK;
}
atomic_uint* sem_count_ptr = SEM_TO_ATOMIC_POINTER(sem);
atomic_init(sem_count_ptr, count);
return 0;
}
@ -122,98 +140,97 @@ int sem_unlink(const char*) {
// 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 uint32_t* sem) {
volatile int32_t* ptr = reinterpret_cast<volatile int32_t*>(sem);
uint32_t shared = (*sem & SEMCOUNT_SHARED_MASK);
uint32_t old_value, new_value;
int ret;
static int __sem_dec(atomic_uint* sem_count_ptr) {
unsigned int old_value = atomic_load_explicit(sem_count_ptr, memory_order_relaxed);
unsigned int shared = old_value & SEMCOUNT_SHARED_MASK;
// Use memory_order_seq_cst in atomic_compare_exchange operation to ensure all
// memory access made by other threads can be seen in current thread.
// An acquire fence may be sufficient, but it is still in discussion whether
// POSIX semaphores should provide sequential consistency.
do {
old_value = (*sem & SEMCOUNT_VALUE_MASK);
ret = SEMCOUNT_TO_VALUE(old_value);
if (ret < 0) {
if (SEMCOUNT_TO_VALUE(old_value) < 0) {
break;
}
} while (!atomic_compare_exchange_weak(sem_count_ptr, &old_value,
SEMCOUNT_DECREMENT(old_value) | shared));
new_value = SEMCOUNT_DECREMENT(old_value);
} while (__bionic_cmpxchg((old_value|shared), (new_value|shared), ptr) != 0);
return ret;
return SEMCOUNT_TO_VALUE(old_value);
}
// 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 uint32_t* sem) {
volatile int32_t* ptr = reinterpret_cast<volatile int32_t*>(sem);
uint32_t shared = (*sem & SEMCOUNT_SHARED_MASK);
uint32_t old_value, new_value;
int ret;
static int __sem_trydec(atomic_uint* sem_count_ptr) {
unsigned int old_value = atomic_load_explicit(sem_count_ptr, memory_order_relaxed);
unsigned int shared = old_value & SEMCOUNT_SHARED_MASK;
// Use memory_order_seq_cst in atomic_compare_exchange operation to ensure all
// memory access made by other threads can be seen in current thread.
// An acquire fence may be sufficient, but it is still in discussion whether
// POSIX semaphores should provide sequential consistency.
do {
old_value = (*sem & SEMCOUNT_VALUE_MASK);
ret = SEMCOUNT_TO_VALUE(old_value);
if (ret <= 0) {
if (SEMCOUNT_TO_VALUE(old_value) <= 0) {
break;
}
} while (!atomic_compare_exchange_weak(sem_count_ptr, &old_value,
SEMCOUNT_DECREMENT(old_value) | shared));
new_value = SEMCOUNT_DECREMENT(old_value);
} while (__bionic_cmpxchg((old_value|shared), (new_value|shared), ptr) != 0);
return ret;
return SEMCOUNT_TO_VALUE(old_value);
}
// "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 uint32_t* sem) {
volatile int32_t* ptr = reinterpret_cast<volatile int32_t*>(sem);
uint32_t shared = (*sem & SEMCOUNT_SHARED_MASK);
uint32_t old_value, new_value;
int ret;
static int __sem_inc(atomic_uint* sem_count_ptr) {
unsigned int old_value = atomic_load_explicit(sem_count_ptr, memory_order_relaxed);
unsigned int shared = old_value & SEMCOUNT_SHARED_MASK;
unsigned int new_value;
// Use memory_order_seq_cst in atomic_compare_exchange operation to ensure all
// memory access made before can be seen in other threads.
// A release fence may be sufficient, but it is still in discussion whether
// POSIX semaphores should provide sequential consistency.
do {
old_value = (*sem & SEMCOUNT_VALUE_MASK);
ret = SEMCOUNT_TO_VALUE(old_value);
// Can't go higher than SEM_VALUE_MAX.
if (ret == SEM_VALUE_MAX) {
if (SEMCOUNT_TO_VALUE(old_value) == SEM_VALUE_MAX) {
break;
}
// If the counter is negative, go directly to +1, otherwise just increment.
if (ret < 0) {
new_value = SEMCOUNT_ONE;
// If the counter is negative, go directly to one, otherwise just increment.
if (SEMCOUNT_TO_VALUE(old_value) < 0) {
new_value = SEMCOUNT_ONE | shared;
} else {
new_value = SEMCOUNT_INCREMENT(old_value);
new_value = SEMCOUNT_INCREMENT(old_value) | shared;
}
} while (__bionic_cmpxchg((old_value|shared), (new_value|shared), ptr) != 0);
} while (!atomic_compare_exchange_weak(sem_count_ptr, &old_value,
new_value));
return ret;
return SEMCOUNT_TO_VALUE(old_value);
}
int sem_wait(sem_t* sem) {
uint32_t shared = SEM_GET_SHARED(sem);
atomic_uint* sem_count_ptr = SEM_TO_ATOMIC_POINTER(sem);
unsigned int shared = SEM_GET_SHARED(sem_count_ptr);
while (true) {
if (__sem_dec(&sem->count) > 0) {
ANDROID_MEMBAR_FULL();
if (__sem_dec(sem_count_ptr) > 0) {
return 0;
}
__futex_wait_ex(&sem->count, shared, shared|SEMCOUNT_MINUS_ONE, NULL);
__futex_wait_ex(sem_count_ptr, shared, shared | SEMCOUNT_MINUS_ONE, NULL);
}
}
int sem_timedwait(sem_t* sem, const timespec* abs_timeout) {
atomic_uint* sem_count_ptr = SEM_TO_ATOMIC_POINTER(sem);
// 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();
if (__sem_trydec(sem_count_ptr) > 0) {
return 0;
}
@ -223,7 +240,7 @@ int sem_timedwait(sem_t* sem, const timespec* abs_timeout) {
return -1;
}
uint32_t shared = SEM_GET_SHARED(sem);
unsigned int shared = SEM_GET_SHARED(sem_count_ptr);
while (true) {
// POSIX mandates CLOCK_REALTIME here.
@ -234,13 +251,12 @@ int sem_timedwait(sem_t* sem, const timespec* abs_timeout) {
}
// 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();
if (__sem_dec(sem_count_ptr) > 0) {
break;
}
// Contention detected. Wait for a wakeup event.
int ret = __futex_wait_ex(&sem->count, shared, shared|SEMCOUNT_MINUS_ONE, &ts);
int ret = __futex_wait_ex(sem_count_ptr, shared, shared | SEMCOUNT_MINUS_ONE, &ts);
// Return in case of timeout or interrupt.
if (ret == -ETIMEDOUT || ret == -EINTR) {
@ -252,13 +268,13 @@ int sem_timedwait(sem_t* sem, const timespec* abs_timeout) {
}
int sem_post(sem_t* sem) {
uint32_t shared = SEM_GET_SHARED(sem);
atomic_uint* sem_count_ptr = SEM_TO_ATOMIC_POINTER(sem);
unsigned int shared = SEM_GET_SHARED(sem_count_ptr);
ANDROID_MEMBAR_FULL();
int old_value = __sem_inc(&sem->count);
int old_value = __sem_inc(sem_count_ptr);
if (old_value < 0) {
// Contention on the semaphore. Wake up all waiters.
__futex_wake_ex(&sem->count, shared, INT_MAX);
__futex_wake_ex(sem_count_ptr, shared, INT_MAX);
} else if (old_value == SEM_VALUE_MAX) {
// Overflow detected.
errno = EOVERFLOW;
@ -269,8 +285,8 @@ int sem_post(sem_t* sem) {
}
int sem_trywait(sem_t* sem) {
if (__sem_trydec(&sem->count) > 0) {
ANDROID_MEMBAR_FULL();
atomic_uint* sem_count_ptr = SEM_TO_ATOMIC_POINTER(sem);
if (__sem_trydec(sem_count_ptr) > 0) {
return 0;
} else {
errno = EAGAIN;
@ -279,7 +295,12 @@ int sem_trywait(sem_t* sem) {
}
int sem_getvalue(sem_t* sem, int* sval) {
int val = SEMCOUNT_TO_VALUE(sem->count);
atomic_uint* sem_count_ptr = SEM_TO_ATOMIC_POINTER(sem);
// Use memory_order_seq_cst in atomic_load operation.
// memory_order_relaxed may be fine here, but it is still in discussion
// whether POSIX semaphores should provide sequential consistency.
int val = SEMCOUNT_TO_VALUE(atomic_load(sem_count_ptr));
if (val < 0) {
val = 0;
}

2
libc/include/semaphore.h
View File

@ -36,7 +36,7 @@ __BEGIN_DECLS
struct timespec;
typedef struct {
volatile unsigned int count;
unsigned int count;
#ifdef __LP64__
int __reserved[3];
#endif