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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 "Futex.h"
#include "include/sys/syscall.h" // For syscall numbers.
#include "include/threads.h" // For call_once related type definition.
#include "src/__support/CPP/atomic.h"
#include "src/__support/OSUtil/syscall.h" // For syscall functions.
#include "src/__support/common.h"
#include "src/threads/call_once.h"
#include "src/threads/linux/Futex.h"
#include <limits.h>
#include <linux/futex.h>
namespace __llvm_libc {
static constexpr FutexWordType START = 0x11;
static constexpr FutexWordType WAITING = 0x22;
static constexpr FutexWordType FINISH = 0x33;
static constexpr once_flag ONCE_FLAG_INIT_VAL = ONCE_FLAG_INIT;
LLVM_LIBC_FUNCTION(void, call_once,
(once_flag * flag, __call_once_func_t func)) {
auto *futex_word = reinterpret_cast<cpp::Atomic<FutexWordType> *>(flag);
static_assert(sizeof(*futex_word) == sizeof(once_flag));
FutexWordType not_called = ONCE_FLAG_INIT_VAL.__word;
// 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 (futex_word->compare_exchange_strong(not_called, START)) {
func();
auto status = futex_word->exchange(FINISH);
if (status == WAITING) {
__llvm_libc::syscall_impl(SYS_futex, &futex_word->val, FUTEX_WAKE_PRIVATE,
INT_MAX, // Wake all waiters.
0, 0, 0);
}
return;
}
FutexWordType status = START;
if (futex_word->compare_exchange_strong(status, WAITING) ||
status == WAITING) {
__llvm_libc::syscall_impl(
SYS_futex, &futex_word->val, FUTEX_WAIT_PRIVATE,
WAITING, // Block only if status is still |WAITING|.
0, 0, 0);
}
}
} // namespace __llvm_libc
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