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