call_once.cpp
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//===-- 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 "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;
void LLVM_LIBC_ENTRYPOINT(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