libc/src/__support/threads/linux/callonce.cpp

   1 //===-- Linux implementation of the callonce 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 "futex_word.h"
  10
  11 #include "src/__support/CPP/atomic.h"
  12 #include "src/__support/OSUtil/syscall.h" // For syscall functions.
  13 #include "src/__support/threads/callonce.h"
  14
  15 #include <limits.h>
  16 #include <linux/futex.h>
  17 #include <sys/syscall.h> // For syscall numbers.
  18
  19 namespace LIBC_NAMESPACE {
  20
  21 static constexpr FutexWordType NOT_CALLED = 0x0;
  22 static constexpr FutexWordType START = 0x11;
  23 static constexpr FutexWordType WAITING = 0x22;
  24 static constexpr FutexWordType FINISH = 0x33;
  25
  26 int callonce(CallOnceFlag *flag, CallOnceCallback *func) {
  27   auto *futex_word = reinterpret_cast<cpp::Atomic<FutexWordType> *>(flag);
  28
  29   FutexWordType not_called = NOT_CALLED;
  30
  31   // The call_once call can return only after the called function |func|
  32   // returns. So, we use futexes to synchronize calls with the same flag value.
  33   if (futex_word->compare_exchange_strong(not_called, START)) {
  34     func();
  35     auto status = futex_word->exchange(FINISH);
  36     if (status == WAITING) {
  37       LIBC_NAMESPACE::syscall_impl<long>(FUTEX_SYSCALL_ID, &futex_word->val,
  38                                          FUTEX_WAKE_PRIVATE,
  39                                          INT_MAX, // Wake all waiters.
  40                                          0, 0, 0);
  41     }
  42     return 0;
  43   }
  44
  45   FutexWordType status = START;
  46   if (futex_word->compare_exchange_strong(status, WAITING) ||
  47       status == WAITING) {
  48     LIBC_NAMESPACE::syscall_impl<long>(
  49         FUTEX_SYSCALL_ID, &futex_word->val, FUTEX_WAIT_PRIVATE,
  50         WAITING, // Block only if status is still |WAITING|.
  51         0, 0, 0);
  52   }
  53
  54   return 0;
  55 }
  56
  57 } // namespace LIBC_NAMESPACE