michael_scott_queue.hpp

//
// Copyright (c) 2018-2020 Manuel Pöter.
// Licensed under the MIT License. See LICENSE file in the project root for full license information.
//
 
#ifndef XENIUM_MICHAEL_SCOTT_QUEUE_HPP
#define XENIUM_MICHAEL_SCOTT_QUEUE_HPP
 
#include <xenium/acquire_guard.hpp>
#include <xenium/backoff.hpp>
#include <xenium/parameter.hpp>
#include <xenium/policy.hpp>
 
#ifdef _MSC_VER
#pragma warning(push)
#pragma warning(disable: 4324) // structure was padded due to alignment specifier
#endif
 
namespace xenium {
template <class T, class... Policies>
class michael_scott_queue {
public:
  using value_type = T;
  using reclaimer = parameter::type_param_t<policy::reclaimer, parameter::nil, Policies...>;
  using backoff = parameter::type_param_t<policy::backoff, no_backoff, Policies...>;
 
  template <class... NewPolicies>
  using with = michael_scott_queue<T, NewPolicies..., Policies...>;
 
  static_assert(parameter::is_set<reclaimer>::value, "reclaimer policy must be specified");
 
  michael_scott_queue();
  ~michael_scott_queue();

  void push(T value);

  [[nodiscard]] bool try_pop(T &result);
 
private:
  struct node;
 
  using concurrent_ptr = typename reclaimer::template concurrent_ptr<node, 0>;
  using marked_ptr = typename concurrent_ptr::marked_ptr;
  using guard_ptr = typename concurrent_ptr::guard_ptr;
  
  struct node : reclaimer::template enable_concurrent_ptr<node>
  {
    node() : value() {};
    node(T&& v) : value(std::move(v)) {}
 
    T value;
    concurrent_ptr next;
  };
  
  alignas(64) concurrent_ptr head;
  alignas(64) concurrent_ptr tail;
};
 
template <class T, class... Policies>
michael_scott_queue<T, Policies...>::michael_scott_queue()
{
  auto n = new node();
  head.store(n, std::memory_order_relaxed);
  tail.store(n, std::memory_order_relaxed);
}
 
template <class T, class... Policies>
michael_scott_queue<T, Policies...>::~michael_scott_queue()
{
  // (1) - this acquire-load synchronizes-with the release-CAS (11)
  auto n = head.load(std::memory_order_acquire);
  while (n)
  {
    // (2) - this acquire-load synchronizes-with the release-CAS (6)
    auto next = n->next.load(std::memory_order_acquire);
    delete n.get();
    n = next;
  }
}
 
template <class T, class... Policies>
void michael_scott_queue<T, Policies...>::push(T value)
{
  node* n = new node(std::move(value));
 
  backoff backoff;
 
  guard_ptr t;
  for (;;)
  {
    // Get the old tail pointer.
    // (3) - this acquire-load synchronizes-with the release-CAS (5, 7, 10)
    t.acquire(tail, std::memory_order_acquire);
 
    // Help update the tail pointer if needed.
    // (4) - this acquire-load synchronizes-with the release-CAS (6)
    auto next = t->next.load(std::memory_order_acquire);
    if (next.get() != nullptr)
    {
      marked_ptr expected(t.get());
      // (5) - this release-CAS synchronizes-with the acquire-load (3)
      tail.compare_exchange_weak(expected, next, std::memory_order_release, std::memory_order_relaxed);
      continue;
    }
 
    // Attempt to link in the new element.
    marked_ptr null{};
    // (6) - this release-CAS synchronizes-with the acquire-load (2, 4, 9).
    if (t->next.compare_exchange_weak(null, n, std::memory_order_release, std::memory_order_relaxed))
      break;
 
    backoff();
  }
 
  // Swing the tail to the new element.
  marked_ptr expected = t.get();
  // (7) - this release-CAS synchronizes-with the acquire-load (3)
  tail.compare_exchange_strong(expected, n, std::memory_order_release, std::memory_order_relaxed);
}
 
template <class T, class... Policies>
bool michael_scott_queue<T, Policies...>::try_pop(T &result)
{
  backoff backoff;
 
  guard_ptr h;
  for (;;)
  {
    // Get the old head and tail elements.
    // (8) - this acquire-load synchronizes-with the release-CAS (11)
    h.acquire(head, std::memory_order_acquire);
 
    // Get the head element's successor.
    // (9) - this acquire-load synchronizes-with the release-CAS (6).
    auto next = acquire_guard(h->next, std::memory_order_acquire);
    if (head.load(std::memory_order_relaxed).get() != h.get())
      continue;
 
    // If the head (dummy) element is the only one, return false to signal that
    // the operation has failed (no element has been returned).
    if (next.get() == nullptr) 
      return false;
 
    marked_ptr t = tail.load(std::memory_order_relaxed);
 
    // There are multiple elements. Help update tail if needed.
    if (h.get() == t.get())
    {
      // (10) - this release-CAS synchronizes-with the acquire-load (3)
      tail.compare_exchange_weak(t, next, std::memory_order_release, std::memory_order_relaxed);
      continue;
    }
 
    // Attempt to update the head pointer so that it points to the new dummy node.
    marked_ptr expected(h.get());
    // (11) - this release-CAS synchronizes-with the acquire-load (1, 8)
    if (head.compare_exchange_weak(expected, next, std::memory_order_release, std::memory_order_relaxed))
    {
      // return the data of head's successor; it is the new dummy node.
      result = std::move(next->value);
      break;
    }
 
    backoff();
  }
 
  // The old dummy node has been unlinked, so reclaim it.
  h.reclaim();
 
  return true;
}
}
 
#ifdef _MSC_VER
#pragma warning(pop)
#endif
 
#endif