The Tor Browser: security/sandbox/chromium/base/lazy

     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.

     5 #include "base/lazy_instance.h"

     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"

    13 namespace base {

    14 namespace internal {

    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;

    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 }

    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);

    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);

    53   // Make sure that the lazily instantiated object will get destroyed at exit.

    54   if (dtor)

    55     AtExitManager::RegisterCallback(dtor, lazy_instance);

    56 }

    58 }  // namespace internal

    59 }  // namespace base

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