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      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.
     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.
     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.
     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.
     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.
     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.
     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