[section:example_reference_counters Reference counting]

The purpose of a ['reference counter] is to count the number
of pointers to an object. The object can be destroyed as
soon as the reference counter reaches zero.

[section Implementation]

[c++]
  
  #include <boost/intrusive_ptr.hpp>
  #include <boost/atomic.hpp>
   
  class X {
  public:
    typedef boost::intrusive_ptr<X> pointer;
    X() : refcount_(0) {}
  
  private:
    mutable boost::atomic<int> refcount_;
    friend void intrusive_ptr_add_ref(const X * x)
    {
      x->refcount_.fetch_add(1, boost::memory_order_relaxed);
    }
    friend void intrusive_ptr_release(const X * x)
    {
      if (x->refcount_.fetch_sub(1, boost::memory_order_release)==1) {
        boost::atomic_thread_fence(boost::memory_order_acquire);
        delete x;
      }
    }
  };

[endsect]

[section Usage]

[c++]
  
  X::pointer x=new X;

[endsect]

[section Discussion]

Increasing the reference counter can always be done with
[^memory_order_relaxed]: New references to an object can only
be formed from an existing reference, and passing an existing
reference from one thread to another must already provide any
required synchronization.

It is important to enforce any possible access to the object in
one thread (through an existing reference) to ['happen before]
deleting the object in a different thread. This is achieved
by a "release" operation after dropping a reference (any
access to the object through this reference must obviously
happened before), and an "acquire" operation before
deleting the object.

It would be possible to use [^memory_order_acq_rel] for the
[^fetch_sub] operation, but this results in unneeded "acquire"
operations when the reference counter does not yet reach zero
and may impose a performance penalty.

[endsect]

[endsect]

[section:example_reference_counters_weak Reference counting with weak references]

The purpose of a ['reference counter] is to count the number
of pointers to an object. The object can be destroyed as
soon as the reference counter reaches zero. There may be
"weak references" to the object that do not count towards preventing
the object from being freed. "True" references may be formed from
"weak" references unless the object has been deleted already.

FIXME: The point to make here is that upgrading "weak" to "full"
references requires memory_order_acquire.

[endsect]

[section:example_spinlock Spinlock]

The purpose of a ['spin lock] is to prevent multiple threads
from concurrently accessing a shared data structure. In contrast
to a mutex, threads will busy-wait and waste CPU cycles instead
of yielding the CPU to another thread. ['Do not use spinlocks
unless you are certain that you understand the consequences.]

[section Implementation]

[c++]
  
  #include <boost/atomic.hpp>
   
  class spinlock {
  private:
    typedef enum {Locked, Unlocked} LockState;
    boost::atomic<LockState> state_;
  
  public:
    spinlock() : state_(Unlocked) {}
    
    lock()
    {
      while(state_.exchange(Locked, boost::memory_order_acquire)==Locked) ;
    }
    unlock()
    {
      state_.store(Unlocked, boost::memory_order_release);
    }
  };

[endsect]

[section Usage]

[c++]
  
  spinlock s;
  
  s.lock();
  // access data structure here
  s.unlock();

[endsect]

[section Discussion]

The purpose of the spinlock is to make sure that one access
to the shared data structure always strictly "happens before"
another. The usage of acquire/release in lock/unlock is required
and sufficient to guarantee this ordering.

It would be correct to write the "lock" operation in the following
way:

[c++]
  
  lock()
  {
    while(state_.exchange(Locked, boost::memory_order_relaxed)==Locked) ;
    atomic_thread_fence(boost::memory_order_acquire);
  }

This "optimization" is however a) useless and b) may in fact hurt:
a) Since the thread will be busily spinning on a blocked spinlock,
it does not matter if it will waste the CPU cycles with just
"exchange" operations or with both useless "exchange" and "acquire"
operations. b) A tight "exchange" loop without any
processor-synchronizing instruction introduced through an "acquire"
operation will on some systems monopolize the memory subsystem
and degrade the performance of other system components.

[endsect]

[endsect]

[section:singleton Double-checked singleton pattern]

The purpose of the ['double-checked singleton pattern] is to ensure
that at most one instance of a particular object is created.
If one instance has been created already, access to the existing
object should be as light-weight as possible.

[section Implementation]

[c++]
  
  #include <boost/atomic.hpp>
  #include <boost/thread/mutex.hpp>
  
  class X {
  public:
    static X * instance()
    {
      X * tmp=instance_.load(boost::memory_order_consume);
      if (!tmp) {
        boost::mutex::scoped_lock l(instantiation_mutex);
        tmp=instance_.load(boost::memory_order_consume);
        if (!tmp) {
          tmp=new X;
          instance_.store(tmp, boost::memory_order_release);
        }
      }
      return tmp;
    }
  private:
    static boost::atomic<X *> instance_;
    static boost::mutex instantion_mutex;
  }
  
  boost::atomic<X *> X::instance_(0);

[endsect]

[section Usage]

[c++]
  
  X * x=X::instance();
  // dereference x

[endsect]

[section Discussion]

The mutex makes sure that only one instance of the object is
ever created. The [^instance] method must make sure that any
dereference of the object strictly "happens after" creating
the instance in another thread. The use of [^memory_order_release]
after creating and initializing the object and [^memory_order_consume]
before dereferencing the object provides this guarantee.

It would be permissible to use [^memory_order_acquire] instead of
[^memory_order_consume], but this provides a stronger guarantee
than is required since only operations depending on the value of
the pointer need to be ordered.

[endsect]

[endsect]

[section:example_ringbuffer Wait-free ring buffer]

A ['wait-free ring buffer] provides a mechanism for relaying objects
from one single "producer" thread to one single "consumer" thread without
any locks. The operations on this data structure are "wait-free" which
means that each operation finishes within a constant number of steps.
This makes this data structure suitable for use in hard real-time systems
or for communication with interrupt/signal handlers.

[section Implementation]

[c++]
  
  #include <boost/atomic.hpp>
  
  template<typename T, size_t Size>
  class ringbuffer {
  public:
    ringbuffer() : head_(0), tail_(0) {}
    
    bool push(const T & value)
    {
      size_t head=head_.load(boost::memory_order_relaxed);
      size_t next_head=next(head);
      if (next_head==tail_.load(boost::memory_order_acquire)) return false;
      ring_[head]=value;
      head_.store(next_head, boost::memory_order_release);
      return true;
    }
    bool pop(T & value)
    {
      size_t tail=tail_.load(boost::memory_order_relaxed);
      if (tail==head_.load(boost::memory_order_acquire)) return false;
      value=ring_[tail];
      tail_.store(next(tail), boost::memory_odrder_release));
      return true;
    }
  private:
    size_t next(size_t current) {return current+1;}
    T ring_[Size];
    boost::atomic<size_t> head_, tail_;
  }

[endsect]

[section Usage]

[c++]
  
  ringbuffer<int, 32> r;
  
  // try to insert an element
  if (r.push(42)) { /* succeeded */ }
  else { /* buffer full */ }

  // try to retrieve an element
  int value;
  if (r.pop(value)) { /* succeeded */ }
  else { /* buffer empty */ }

[endsect]

[section Discussion]

The implementation makes sure that the ring indices do
not "lap-around" each other to ensure that no elements
are either lost or read twice.

Furthermore it must guarantee that read-access to a
particular object in [^pop] "happens after" it has been
written in [^push]. This is achieved by writing [^head_ ]
with "release" and reading it with "acquire". Conversely
the implementation also ensures that read access to
a particular ring element "happens before" before
rewriting this element with a new value by accessing [^tail_]
with appropriate ordering constraints.

[endsect]

[endsect]

[section:mp_queue Wait-free multi-producer queue]

The purpose of the ['wait-free multi-producer queue] is to allow
an arbitrary number of producers to enqueue objects which are
retrieved and processed in FIFO order by a single consumer.

[section Implementation]

[c++]

  template<typename T>
  class waitfree_queue {
  public:
    struct node {
      T data;
      node * next;
    }
    void push(const T &data)
    {
      node * n=new node;
      n.data=data;
      node * stale_head=head_.load(boost::memory_order_relaxed);
      do {
        node->next=stale_head;
      } while(!head_.compare_exchange_weak(stale_head, node, boost::memory_order_release);
    }
    
    node * pop_all(void)
    {
      T *last=pop_all_reverse(), *first=0;
      while(last) {
        T * tmp=last;
        last=last->next;
        tmp->next=first;
        first=tmp;
      }
      return first;
    }
    
    waitfree_queue() : head_(0) {}
    
    // alternative interface if ordering is of no importance
    node * pop_all_reverse(void)
    {
      return head_.exchange(0, boost::memory_order_consume);
    }
  private:
    boost::atomic<node *> head_;
  }

[endsect]

[section Usage]

[c++]
  
  waitfree_queue<int> q;
  
  // insert elements
  q.push(42);
  q.push(2);
  
  // pop elements
  waitfree_queue<int>::node * x=q.pop_all()
  while(x) {
    X * tmp=x;
    x=x->next;
    // process tmp->data, probably delete it afterwards
    delete tmp;
  }

[endsect]

[section Discussion]

The implementation guarantees that all objects enqueued are
processed in the order they were enqueued by building a singly-linked
list of object in reverse processing order. The queue is atomically
emptied by the consumer and brought into correct order.

It must be guaranteed that any access to an object to be enqueued
by the producer "happens before" any access by the consumer. This
is assured by inserting objects into the list with ['release] and
dequeuing them with ['consume] memory order. It is not
necessary to use ['acquire] memory order in [^waitfree_queue::pop_all]
because all operations involved depend on the value of
the atomic pointer through dereference

[endsect]

[endsect]
