once.cc 1.55 KB
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// Copyright 2012 the V8 project authors. All rights reserved.
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// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.
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#include "src/base/once.h"
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#ifdef _WIN32
#include <windows.h>
#else
#include <sched.h>
#endif

namespace v8 {
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namespace base {
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void CallOnceImpl(OnceType* once, std::function<void()> init_func) {
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  // Fast path. The provided function was already executed.
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  if (once->load(std::memory_order_acquire) == ONCE_STATE_DONE) {
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    return;
  }

  // The function execution did not complete yet. The once object can be in one
  // of the two following states:
  //   - UNINITIALIZED: We are the first thread calling this function.
  //   - EXECUTING_FUNCTION: Another thread is already executing the function.
  //
  // First, try to change the state from UNINITIALIZED to EXECUTING_FUNCTION
  // atomically.
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  uint8_t expected = ONCE_STATE_UNINITIALIZED;
  if (once->compare_exchange_strong(expected, ONCE_STATE_EXECUTING_FUNCTION,
                                    std::memory_order_acq_rel)) {
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    // We are the first thread to call this function, so we have to call the
    // function.
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    init_func();
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    once->store(ONCE_STATE_DONE, std::memory_order_release);
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  } else {
    // Another thread has already started executing the function. We need to
    // wait until it completes the initialization.
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    while (once->load(std::memory_order_acquire) ==
           ONCE_STATE_EXECUTING_FUNCTION) {
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#ifdef _WIN32
      ::Sleep(0);
#else
      sched_yield();
#endif
    }
  }
}

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}  // namespace base
}  // namespace v8