| //===-- Linux implementation of the call_once function --------------------===// |
| // |
| // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions. |
| // See https://llvm.org/LICENSE.txt for license information. |
| // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception |
| // |
| //===----------------------------------------------------------------------===// |
| |
| #include "src/threads/call_once.h" |
| #include "config/linux/syscall.h" // For syscall functions. |
| #include "include/sys/syscall.h" // For syscall numbers. |
| #include "include/threads.h" // For call_once related type definition. |
| #include "src/__support/common.h" |
| #include "src/threads/linux/thread_utils.h" |
| |
| #include <limits.h> |
| #include <linux/futex.h> |
| #include <stdatomic.h> |
| |
| namespace __llvm_libc { |
| |
| static constexpr unsigned START = 0x11; |
| static constexpr unsigned WAITING = 0x22; |
| static constexpr unsigned FINISH = 0x33; |
| |
| LLVM_LIBC_FUNCTION(void, call_once, |
| (once_flag * flag, __call_once_func_t func)) { |
| FutexData *futex_word = reinterpret_cast<FutexData *>(flag); |
| unsigned int not_called = ONCE_FLAG_INIT; |
| |
| // The C standard wording says: |
| // |
| // The completion of the function func synchronizes with all |
| // previous or subsequent calls to call_once with the same |
| // flag variable. |
| // |
| // What this means is that, the call_once call can return only after |
| // the called function |func| returns. So, we use futexes to synchronize |
| // calls with the same flag value. |
| if (::atomic_compare_exchange_strong(futex_word, ¬_called, START)) { |
| func(); |
| auto status = ::atomic_exchange(futex_word, FINISH); |
| if (status == WAITING) { |
| __llvm_libc::syscall(SYS_futex, futex_word, FUTEX_WAKE_PRIVATE, |
| INT_MAX, // Wake all waiters. |
| 0, 0, 0); |
| } |
| return; |
| } |
| |
| unsigned int status = START; |
| if (::atomic_compare_exchange_strong(futex_word, &status, WAITING) || |
| status == WAITING) { |
| __llvm_libc::syscall(SYS_futex, futex_word, FUTEX_WAIT_PRIVATE, |
| WAITING, // Block only if status is still |WAITING|. |
| 0, 0, 0); |
| } |
| } |
| |
| } // namespace __llvm_libc |