在一個pthread_once方法內又再次呼叫了這個pthread_once導致死鎖。
分析下這個pthread_once的原始碼:
可以看到這個pthread_once_t結構體就是一個整形數字加自旋鎖。
int
___pthread_once (pthread_once_t *once_control, void (*init_routine) (void))
{
/* Fast path. See __pthread_once_slow. */
int val;
val = atomic_load_acquire (once_control);
if (__glibc_likely ((val & __PTHREAD_ONCE_DONE) != 0)) // 這裡判斷如果已經執行完成就返回0
return 0;
else
return __pthread_once_slow (once_control, init_routine);
}
static int
__attribute__ ((noinline))
__pthread_once_slow (pthread_once_t *once_control, void (*init_routine) (void))
{
while (1)
{
int val, newval;
/* CAS把狀態改為__PTHREAD_ONCE_INPROGRESS */
/* We need acquire memory order for this load because if the value
signals that initialization has finished, we need to see any
data modifications done during initialization. */
val = atomic_load_acquire (once_control);
do
{
/* Check if the initialization has already been done. */
if (__glibc_likely ((val & __PTHREAD_ONCE_DONE) != 0))
return 0;
/* We try to set the state to in-progress and having the current
fork generation. We don't need atomic accesses for the fork
generation because it's immutable in a particular process, and
forked child processes start with a single thread that modified
the generation. */
newval = __fork_generation | __PTHREAD_ONCE_INPROGRESS;
/* We need acquire memory order here for the same reason as for the
load from once_control above. */
}
while (__glibc_unlikely (!atomic_compare_exchange_weak_acquire (
once_control, &val, newval)));
/* Check if another thread already runs the initializer. */
if ((val & __PTHREAD_ONCE_INPROGRESS) != 0)
{
/* 修改失敗,說明其他執行緒在修改,直接等待 */
/* Check whether the initializer execution was interrupted by a
fork. We know that for both values, __PTHREAD_ONCE_INPROGRESS
is set and __PTHREAD_ONCE_DONE is not. */
if (val == newval)
{
/* Same generation, some other thread was faster. Wait and
retry. */
futex_wait_simple ((unsigned int *) once_control,
(unsigned int) newval, FUTEX_PRIVATE);
continue;
}
}
/* This thread is the first here. Do the initialization.
Register a cleanup handler so that in case the thread gets
interrupted the initialization can be restarted. */
pthread_cleanup_combined_push (clear_once_control, once_control);
/* 呼叫使用者傳入的方法 */
init_routine ();
pthread_cleanup_combined_pop (0);
/* Mark *once_control as having finished the initialization. We need
release memory order here because we need to synchronize with other
threads that want to use the initialized data. */
/* 修改狀態為__PTHREAD_ONCE_DONE */
atomic_store_release (once_control, __PTHREAD_ONCE_DONE);
/* Wake up all other threads. */
/* 喚醒其他執行緒 */
futex_wake ((unsigned int *) once_control, INT_MAX, FUTEX_PRIVATE);
break;
}
return 0;
}