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 #include "base/third_party/dynamic_annotations/dynamic_annotations.h" 12 13 namespace base { 14 namespace internal { 15 16 // TODO(joth): This function could be shared with Singleton, in place of its 17 // WaitForInstance() call. 18 bool NeedsLazyInstance(subtle::AtomicWord* state) { 19 // Try to create the instance, if we're the first, will go from 0 to 20 // kLazyInstanceStateCreating, otherwise we've already been beaten here. 21 // The memory access has no memory ordering as state 0 and 22 // kLazyInstanceStateCreating have no associated data (memory barriers are 23 // all about ordering of memory accesses to *associated* data). 24 if (subtle::NoBarrier_CompareAndSwap(state, 0, 25 kLazyInstanceStateCreating) == 0) 26 // Caller must create instance 27 return true; 28 29 // It's either in the process of being created, or already created. Spin. 30 // The load has acquire memory ordering as a thread which sees 31 // state_ == STATE_CREATED needs to acquire visibility over 32 // the associated data (buf_). Pairing Release_Store is in 33 // CompleteLazyInstance(). 34 while (subtle::Acquire_Load(state) == kLazyInstanceStateCreating) { 35 PlatformThread::YieldCurrentThread(); 36 } 37 // Someone else created the instance. 38 return false; 39 } 40 41 void CompleteLazyInstance(subtle::AtomicWord* state, 42 subtle::AtomicWord new_instance, 43 void* lazy_instance, 44 void (*dtor)(void*)) { 45 // See the comment to the corresponding HAPPENS_AFTER in Pointer(). 46 ANNOTATE_HAPPENS_BEFORE(state); 47 48 // Instance is created, go from CREATING to CREATED. 49 // Releases visibility over private_buf_ to readers. Pairing Acquire_Load's 50 // are in NeedsInstance() and Pointer(). 51 subtle::Release_Store(state, new_instance); 52 53 // Make sure that the lazily instantiated object will get destroyed at exit. 54 if (dtor) 55 AtExitManager::RegisterCallback(dtor, lazy_instance); 56 } 57 58 } // namespace internal 59 } // namespace base 60
