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 
CallOnceImpl(OnceType * once,PointerArgFunction init_func,void * arg)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