1 // Copyright (c) 2011 The Chromium 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 "base/lazy_instance.h" 6 7 #include "base/at_exit.h" 8 #include "base/atomicops.h" 9 #include "base/basictypes.h" 10 #include "base/threading/platform_thread.h" 11 12 namespace base { 13 namespace internal { 14 15 // TODO(joth): This function could be shared with Singleton, in place of its 16 // WaitForInstance() call. 17 bool NeedsLazyInstance(subtle::AtomicWord* state) { 18 // Try to create the instance, if we're the first, will go from 0 to 19 // kLazyInstanceStateCreating, otherwise we've already been beaten here. 20 // The memory access has no memory ordering as state 0 and 21 // kLazyInstanceStateCreating have no associated data (memory barriers are 22 // all about ordering of memory accesses to *associated* data). 23 if (subtle::NoBarrier_CompareAndSwap(state, 0, 24 kLazyInstanceStateCreating) == 0) 25 // Caller must create instance 26 return true; 27 28 // It's either in the process of being created, or already created. Spin. 29 // The load has acquire memory ordering as a thread which sees 30 // state_ == STATE_CREATED needs to acquire visibility over 31 // the associated data (buf_). Pairing Release_Store is in 32 // CompleteLazyInstance(). 33 while (subtle::Acquire_Load(state) == kLazyInstanceStateCreating) { 34 PlatformThread::YieldCurrentThread(); 35 } 36 // Someone else created the instance. 37 return false; 38 } 39 40 void CompleteLazyInstance(subtle::AtomicWord* state, 41 subtle::AtomicWord new_instance, 42 void* lazy_instance, 43 void (*dtor)(void*)) { 44 // Instance is created, go from CREATING to CREATED. 45 // Releases visibility over private_buf_ to readers. Pairing Acquire_Load's 46 // are in NeedsInstance() and Pointer(). 47 subtle::Release_Store(state, new_instance); 48 49 // Make sure that the lazily instantiated object will get destroyed at exit. 50 if (dtor) 51 AtExitManager::RegisterCallback(dtor, lazy_instance); 52 } 53 54 } // namespace internal 55 } // namespace base 56
