[英]Why accessing pthread keys' sequence number is not synchronized in glibc's NPTL implementation?
最近,當我研究如何在glibc中實現線程本地存儲時,發現了以下代碼,該代碼實現了API pthread_key_create()
int
__pthread_key_create (key, destr)
pthread_key_t *key;
void (*destr) (void *);
{
/* Find a slot in __pthread_kyes which is unused. */
for (size_t cnt = 0; cnt < PTHREAD_KEYS_MAX; ++cnt)
{
uintptr_t seq = __pthread_keys[cnt].seq;
if (KEY_UNUSED (seq) && KEY_USABLE (seq)
/* We found an unused slot. Try to allocate it. */
&& ! atomic_compare_and_exchange_bool_acq (&__pthread_keys[cnt].seq,
seq + 1, seq))
{
/* Remember the destructor. */
__pthread_keys[cnt].destr = destr;
/* Return the key to the caller. */
*key = cnt;
/* The call succeeded. */
return 0;
}
}
return EAGAIN;
}
__pthread_keys
是所有線程訪問的全局數組。 我不明白為什么其成員seq
的讀取不同步,如下所示:
uintptr_t seq = __pthread_keys[cnt].seq;
盡管稍后修改它已同步。
僅供參考, __pthread_keys
struct pthread_key_struct
是類型為struct pthread_key_struct
的數組,其定義如下:
/* Thread-local data handling. */
struct pthread_key_struct
{
/* Sequence numbers. Even numbers indicated vacant entries. Note
that zero is even. We use uintptr_t to not require padding on
32- and 64-bit machines. On 64-bit machines it helps to avoid
wrapping, too. */
uintptr_t seq;
/* Destructor for the data. */
void (*destr) (void *);
};
提前致謝。
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