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 // This is a low level implementation of atomic semantics for reference
6 // counting. Please use base/memory/ref_counted.h directly instead.
7 //
8 // The implementation includes annotations to avoid some false positives
9 // when using data race detection tools.
10
11 #ifndef BASE_ATOMIC_REF_COUNT_H_
12 #define BASE_ATOMIC_REF_COUNT_H_
13 #pragma once
14
15 #include "base/atomicops.h"
16 #include "base/third_party/dynamic_annotations/dynamic_annotations.h"
17
18 namespace base {
19
20 typedef subtle::Atomic32 AtomicRefCount;
21
22 // Increment a reference count by "increment", which must exceed 0.
AtomicRefCountIncN(volatile AtomicRefCount * ptr,AtomicRefCount increment)23 inline void AtomicRefCountIncN(volatile AtomicRefCount *ptr,
24 AtomicRefCount increment) {
25 subtle::NoBarrier_AtomicIncrement(ptr, increment);
26 }
27
28 // Decrement a reference count by "decrement", which must exceed 0,
29 // and return whether the result is non-zero.
30 // Insert barriers to ensure that state written before the reference count
31 // became zero will be visible to a thread that has just made the count zero.
AtomicRefCountDecN(volatile AtomicRefCount * ptr,AtomicRefCount decrement)32 inline bool AtomicRefCountDecN(volatile AtomicRefCount *ptr,
33 AtomicRefCount decrement) {
34 ANNOTATE_HAPPENS_BEFORE(ptr);
35 bool res = (subtle::Barrier_AtomicIncrement(ptr, -decrement) != 0);
36 if (!res) {
37 ANNOTATE_HAPPENS_AFTER(ptr);
38 }
39 return res;
40 }
41
42 // Increment a reference count by 1.
AtomicRefCountInc(volatile AtomicRefCount * ptr)43 inline void AtomicRefCountInc(volatile AtomicRefCount *ptr) {
44 base::AtomicRefCountIncN(ptr, 1);
45 }
46
47 // Decrement a reference count by 1 and return whether the result is non-zero.
48 // Insert barriers to ensure that state written before the reference count
49 // became zero will be visible to a thread that has just made the count zero.
AtomicRefCountDec(volatile AtomicRefCount * ptr)50 inline bool AtomicRefCountDec(volatile AtomicRefCount *ptr) {
51 return base::AtomicRefCountDecN(ptr, 1);
52 }
53
54 // Return whether the reference count is one. If the reference count is used
55 // in the conventional way, a refrerence count of 1 implies that the current
56 // thread owns the reference and no other thread shares it. This call performs
57 // the test for a reference count of one, and performs the memory barrier
58 // needed for the owning thread to act on the object, knowing that it has
59 // exclusive access to the object.
AtomicRefCountIsOne(volatile AtomicRefCount * ptr)60 inline bool AtomicRefCountIsOne(volatile AtomicRefCount *ptr) {
61 bool res = (subtle::Acquire_Load(ptr) == 1);
62 if (res) {
63 ANNOTATE_HAPPENS_AFTER(ptr);
64 }
65 return res;
66 }
67
68 // Return whether the reference count is zero. With conventional object
69 // referencing counting, the object will be destroyed, so the reference count
70 // should never be zero. Hence this is generally used for a debug check.
AtomicRefCountIsZero(volatile AtomicRefCount * ptr)71 inline bool AtomicRefCountIsZero(volatile AtomicRefCount *ptr) {
72 bool res = (subtle::Acquire_Load(ptr) == 0);
73 if (res) {
74 ANNOTATE_HAPPENS_AFTER(ptr);
75 }
76 return res;
77 }
78
79 } // namespace base
80
81 #endif // BASE_ATOMIC_REF_COUNT_H_
82