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      1 // Copyright 2012 the V8 project authors. All rights reserved.
      2 // Use of this source code is governed by a BSD-style license that can be
      3 // found in the LICENSE file.
      4 
      5 #include "src/base/once.h"
      6 
      7 #ifdef _WIN32
      8 #include <windows.h>
      9 #else
     10 #include <sched.h>
     11 #endif
     12 
     13 #include "src/base/atomicops.h"
     14 
     15 namespace v8 {
     16 namespace base {
     17 
     18 void CallOnceImpl(OnceType* once, PointerArgFunction init_func, void* arg) {
     19   AtomicWord state = Acquire_Load(once);
     20   // Fast path. The provided function was already executed.
     21   if (state == ONCE_STATE_DONE) {
     22     return;
     23   }
     24 
     25   // The function execution did not complete yet. The once object can be in one
     26   // of the two following states:
     27   //   - UNINITIALIZED: We are the first thread calling this function.
     28   //   - EXECUTING_FUNCTION: Another thread is already executing the function.
     29   //
     30   // First, try to change the state from UNINITIALIZED to EXECUTING_FUNCTION
     31   // atomically.
     32   state = Acquire_CompareAndSwap(
     33       once, ONCE_STATE_UNINITIALIZED, ONCE_STATE_EXECUTING_FUNCTION);
     34   if (state == ONCE_STATE_UNINITIALIZED) {
     35     // We are the first thread to call this function, so we have to call the
     36     // function.
     37     init_func(arg);
     38     Release_Store(once, ONCE_STATE_DONE);
     39   } else {
     40     // Another thread has already started executing the function. We need to
     41     // wait until it completes the initialization.
     42     while (state == ONCE_STATE_EXECUTING_FUNCTION) {
     43 #ifdef _WIN32
     44       ::Sleep(0);
     45 #else
     46       sched_yield();
     47 #endif
     48       state = Acquire_Load(once);
     49     }
     50   }
     51 }
     52 
     53 } }  // namespace v8::base
     54