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libc: optimize pthread_once() implementation.

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This patch changes the implementation of pthread_once()
to avoid the use of a single global recursive mutex. This
should also slightly speed up the non-common case where
we have to call the init function, or wait for another
thread to finish the call.

Change-Id: I8a93f4386c56fb89b5d0eb716689c2ce43bdcad9
This commit is contained in:
David 'Digit' Turner 2011-12-07 12:20:44 +01:00
parent 6e9d51701e
commit 6c6de44f04
1 changed files with 62 additions and 10 deletions
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72
libc/bionic/pthread.c
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@ -1931,18 +1931,70 @@ int pthread_once( pthread_once_t* once_control, void (*init_routine)(void) )
{
static pthread_mutex_t once_lock = PTHREAD_RECURSIVE_MUTEX_INITIALIZER;
volatile pthread_once_t* ocptr = once_control;
pthread_once_t value;
pthread_once_t tmp = *ocptr;
ANDROID_MEMBAR_FULL();
if (tmp == PTHREAD_ONCE_INIT) {
pthread_mutex_lock( &once_lock );
if (*ocptr == PTHREAD_ONCE_INIT) {
(*init_routine)();
ANDROID_MEMBAR_FULL();
*ocptr = ~PTHREAD_ONCE_INIT;
}
pthread_mutex_unlock( &once_lock );
/* PTHREAD_ONCE_INIT is 0, we use the following bit flags
*
* bit 0 set -> initialization is under way
* bit 1 set -> initialization is complete
*/
#define ONCE_INITIALIZING (1 << 0)
#define ONCE_COMPLETED (1 << 1)
/* 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 (__likely((*ocptr & ONCE_COMPLETED) != 0)) {
ANDROID_MEMBAR_FULL();
return 0;
}
for (;;) {
/* 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 oldval, newval;
do {
oldval = *ocptr;
if ((oldval & ONCE_COMPLETED) != 0)
break;
newval = oldval | ONCE_INITIALIZING;
} while (__bionic_cmpxchg(oldval, newval, ocptr) != 0);
if ((oldval & ONCE_COMPLETED) != 0) {
/* We detected that COMPLETED was set while in our loop */
ANDROID_MEMBAR_FULL();
return 0;
}
if ((oldval & ONCE_INITIALIZING) == 0) {
/* We got there first, we can jump out of the loop to
* handle the initialization */
break;
}
/* Another thread is running the initialization and hasn't completed
* yet, so wait for it, then try again. */
__futex_wait_ex(ocptr, 0, oldval, NULL);
}
/* call the initialization function. */
(*init_routine)();
/* Do a store_release indicating that initialization is complete */
ANDROID_MEMBAR_FULL();
*ocptr = ONCE_COMPLETED;
/* Wake up any waiters, if any */
__futex_wake_ex(ocptr, 0, INT_MAX);
return 0;
}