1 //===-- Linux implementation of the call_once function --------------------===// 2 // 3 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions. 4 // See https://llvm.org/LICENSE.txt for license information. 5 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception 6 // 7 //===----------------------------------------------------------------------===// 8 9 #include "src/threads/call_once.h" 10 #include "config/linux/syscall.h" // For syscall functions. 11 #include "include/sys/syscall.h" // For syscall numbers. 12 #include "include/threads.h" // For call_once related type definition. 13 #include "src/__support/common.h" 14 #include "src/threads/linux/Futex.h" 15 16 #include <limits.h> 17 #include <linux/futex.h> 18 #include <stdatomic.h> 19 20 namespace __llvm_libc { 21 22 static constexpr unsigned START = 0x11; 23 static constexpr unsigned WAITING = 0x22; 24 static constexpr unsigned FINISH = 0x33; 25 26 LLVM_LIBC_FUNCTION(void, call_once, 27 (once_flag * flag, __call_once_func_t func)) { 28 FutexWord *futex_word = reinterpret_cast<FutexWord *>(flag); 29 unsigned int not_called = ONCE_FLAG_INIT; 30 31 // The C standard wording says: 32 // 33 // The completion of the function func synchronizes with all 34 // previous or subsequent calls to call_once with the same 35 // flag variable. 36 // 37 // What this means is that, the call_once call can return only after 38 // the called function |func| returns. So, we use futexes to synchronize 39 // calls with the same flag value. 40 if (::atomic_compare_exchange_strong(futex_word, ¬_called, START)) { 41 func(); 42 auto status = ::atomic_exchange(futex_word, FINISH); 43 if (status == WAITING) { 44 __llvm_libc::syscall(SYS_futex, futex_word, FUTEX_WAKE_PRIVATE, 45 INT_MAX, // Wake all waiters. 46 0, 0, 0); 47 } 48 return; 49 } 50 51 unsigned int status = START; 52 if (::atomic_compare_exchange_strong(futex_word, &status, WAITING) || 53 status == WAITING) { 54 __llvm_libc::syscall(SYS_futex, futex_word, FUTEX_WAIT_PRIVATE, 55 WAITING, // Block only if status is still |WAITING|. 56 0, 0, 0); 57 } 58 } 59 60 } // namespace __llvm_libc 61