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POWhy accessing pthread keys' sequence number is not synchronized in glibc's NPTL implementation?
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copied!<p>Recently when I look into how the thread-local storage is implemented in glibc, I found the following code, which implements the API <code>pthread_key_create()</code></p> <pre><code>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; } </code></pre> <p><code>__pthread_keys</code> is a global array accessed by all threads. I don't understand why the read of its member <code>seq</code> is not synchronized as in the following:</p> <pre><code>uintptr_t seq = __pthread_keys[cnt].seq; </code></pre> <p>although it is syncrhonized when modified later.</p> <p>FYI, <code>__pthread_keys</code> is an array of type <code>struct pthread_key_struct</code>, which is defined as follows:</p> <pre><code>/* 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 *); }; </code></pre> <p>Thanks in advance.</p>

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