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Merge "Switch pthread_once_t to stdatomic.h."

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This commit is contained in:
Yabin Cui 2015-01-28 21:16:04 +00:00 committed by Gerrit Code Review
commit 9e82c4be35
2 changed files with 32 additions and 47 deletions
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77
libc/bionic/pthread_once.cpp
View File

@ -27,74 +27,59 @@
*/
#include <pthread.h>
#include <stdatomic.h>
#include "private/bionic_atomic_inline.h"
#include "private/bionic_futex.h"
#define ONCE_INITIALIZING (1 << 0)
#define ONCE_COMPLETED (1 << 1)
#define ONCE_INITIALIZATION_NOT_YET_STARTED 0
#define ONCE_INITIALIZATION_UNDERWAY 1
#define ONCE_INITIALIZATION_COMPLETE 2
/* NOTE: this implementation doesn't support a init function that throws a C++ exception
* or calls fork()
*/
int pthread_once(pthread_once_t* once_control, void (*init_routine)(void)) {
volatile pthread_once_t* once_control_ptr = once_control;
static_assert(sizeof(atomic_int) == sizeof(pthread_once_t),
"pthread_once_t should actually be atomic_int in implementation.");
// PTHREAD_ONCE_INIT is 0, we use the following bit flags
// bit 0 set -> initialization is under way
// bit 1 set -> initialization is complete
// We prefer casting to atomic_int instead of declaring pthread_once_t to be atomic_int directly.
// Because using the second method pollutes pthread.h, and causes an error when compiling libcxx.
atomic_int* once_control_ptr = reinterpret_cast<atomic_int*>(once_control);
// First check if the once is already initialized. This will be the common
// case and we want to make this as fast as possible. Note that this still
// requires a load_acquire operation here to ensure that all the
// stores performed by the initialization function are observable on
// this CPU after we exit.
if (__predict_true((*once_control_ptr & ONCE_COMPLETED) != 0)) {
ANDROID_MEMBAR_FULL();
return 0;
}
int old_value = atomic_load_explicit(once_control_ptr, memory_order_acquire);
while (true) {
// Try to atomically set the INITIALIZING flag.
// This requires a cmpxchg loop, and we may need
// to exit prematurely if we detect that
// COMPLETED is now set.
int32_t old_value, new_value;
do {
old_value = *once_control_ptr;
if ((old_value & ONCE_COMPLETED) != 0) {
break;
}
new_value = old_value | ONCE_INITIALIZING;
} while (__bionic_cmpxchg(old_value, new_value, once_control_ptr) != 0);
if ((old_value & ONCE_COMPLETED) != 0) {
// We detected that COMPLETED was set while in our loop.
ANDROID_MEMBAR_FULL();
if (__predict_true(old_value == ONCE_INITIALIZATION_COMPLETE)) {
return 0;
}
if ((old_value & ONCE_INITIALIZING) == 0) {
// We got there first, we can jump out of the loop to handle the initialization.
break;
// Try to atomically set the initialization underway flag. This requires a compare exchange
// in a loop, and we may need to exit prematurely if the initialization is complete.
if (!atomic_compare_exchange_weak_explicit(once_control_ptr, &old_value,
ONCE_INITIALIZATION_UNDERWAY,
memory_order_acquire, memory_order_acquire)) {
continue;
}
// Another thread is running the initialization and hasn't completed
// yet, so wait for it, then try again.
if (old_value == ONCE_INITIALIZATION_NOT_YET_STARTED) {
// We got here first, we can handle the initialization.
(*init_routine)();
// Do a store_release indicating that initialization is complete.
atomic_store_explicit(once_control_ptr, ONCE_INITIALIZATION_COMPLETE, memory_order_release);
// Wake up any waiters, if any.
__futex_wake_ex(once_control_ptr, 0, INT_MAX);
return 0;
}
// The initialization is underway, wait for its finish.
__futex_wait_ex(once_control_ptr, 0, old_value, NULL);
old_value = atomic_load_explicit(once_control_ptr, memory_order_acquire);
}
// Call the initialization function.
(*init_routine)();
// Do a store_release indicating that initialization is complete.
ANDROID_MEMBAR_FULL();
*once_control_ptr = ONCE_COMPLETED;
// Wake up any waiters, if any.
__futex_wake_ex(once_control_ptr, 0, INT_MAX);
return 0;
}

2
libc/include/pthread.h
View File

@ -112,7 +112,7 @@ typedef struct {
typedef int pthread_key_t;
typedef volatile int pthread_once_t;
typedef int pthread_once_t;
#define PTHREAD_ONCE_INIT 0