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// token.h -- lock tokens for gold   -*- C++ -*-

// Copyright 2006, 2007, 2008 Free Software Foundation, Inc.

// Written by Ian Lance Taylor <iant@google.com>.

// This file is part of gold.

// This program is free software; you can redistribute it and/or modify

// it under the terms of the GNU General Public License as published by

// the Free Software Foundation; either version 3 of the License, or

// (at your option) any later version.

// This program is distributed in the hope that it will be useful,

// but WITHOUT ANY WARRANTY; without even the implied warranty of

// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the

// GNU General Public License for more details.

// You should have received a copy of the GNU General Public License

// along with this program; if not, write to the Free Software

// Foundation, Inc., 51 Franklin Street - Fifth Floor, Boston,

// MA 02110-1301, USA.

#ifndef GOLD_TOKEN_H

#define GOLD_TOKEN_H

namespace gold

{

class Condvar;

class Task;

// A list of Tasks, managed through the next_locked_ field in the

// class Task.  We define this class here because we need it in

// Task_token.

class Task_list

{

 public:

  Task_list()

    : head_(NULL), tail_(NULL)

  { }

  ~Task_list()

  { gold_assert(this->head_ == NULL && this->tail_ == NULL); }

  // Return whether the list is empty.

  bool

  empty() const

  { return this->head_ == NULL; }

  // Add T to the head of the list.

  void

  push_front(Task* t);

  // Add T to the end of the list.

  void

  push_back(Task* t);

  // Remove the first Task on the list and return it.  Return NULL if

  // the list is empty.

  Task*

  pop_front();

 private:

  // The start of the list.  NULL if the list is empty.

  Task* head_;

  // The end of the list.  NULL if the list is empty.

  Task* tail_;

};

// We support two basic types of locks, which are both implemented

// using the single class Task_token.

// A write lock may be held by a single Task at a time.  This is used

// to control access to a single shared resource such as an Object.

// A blocker is used to indicate that a Task A must be run after some

// set of Tasks B.  For each of the Tasks B, we increment the blocker

// when the Task is created, and decrement it when the Task is

// completed.  When the count goes to 0, the task A is ready to run.

// There are no shared read locks.  We always read and write objects

// in predictable patterns.  The purpose of the locks is to permit

// some flexibility for the threading system, for cases where the

// execution order does not matter.

// These tokens are only manipulated when the workqueue lock is held

// or when they are first created.  They do not require any locking

// themselves.

class Task_token

{

 public:

  Task_token(bool is_blocker)

    : is_blocker_(is_blocker), blockers_(0), writer_(NULL), waiting_()

  { }

  ~Task_token()

  {

    gold_assert(this->blockers_ == 0);

    gold_assert(this->writer_ == NULL);

  }

  // Return whether this is a blocker.

  bool

  is_blocker() const

  { return this->is_blocker_; }

  // A write lock token uses these methods.

  // Is the token writable?

  bool

  is_writable() const

  {

    gold_assert(!this->is_blocker_);

    return this->writer_ == NULL;

  }

  // Add the task as the token's writer (there may only be one

  // writer).

  void

  add_writer(const Task* t)

  {

    gold_assert(!this->is_blocker_ && this->writer_ == NULL);

    this->writer_ = t;

  }

  // Remove the task as the token's writer.

  void

  remove_writer(const Task* t)

  {

    gold_assert(!this->is_blocker_ && this->writer_ == t);

    this->writer_ = NULL;

  }

  // A blocker token uses these methods.

  // Add a blocker to the token.

  void

  add_blocker()

  {

    gold_assert(this->is_blocker_);

    ++this->blockers_;

    this->writer_ = NULL;

  }

  // Remove a blocker from the token.  Returns true if block count

  // drops to zero.

  bool

  remove_blocker()

  {

    gold_assert(this->is_blocker_ && this->blockers_ > 0);

    --this->blockers_;

    this->writer_ = NULL;

    return this->blockers_ == 0;

  }

  // Is the token currently blocked?

  bool

  is_blocked() const

  {

    gold_assert(this->is_blocker_);

    return this->blockers_ > 0;

  }

  // Both blocker and write lock tokens use these methods.

  // Add T to the list of tasks waiting for this token to be released.

  void

  add_waiting(Task* t)

  { this->waiting_.push_back(t); }

  // Add T to the front of the list of tasks waiting for this token to

  // be released.

  void

  add_waiting_front(Task* t)

  { this->waiting_.push_front(t); }

  // Remove the first Task waiting for this token to be released, and

  // return it.  Return NULL if no Tasks are waiting.

  Task*

  remove_first_waiting()

  { return this->waiting_.pop_front(); }

 private:

  // It makes no sense to copy these.

  Task_token(const Task_token&);

  Task_token& operator=(const Task_token&);

  // Whether this is a blocker token.

  bool is_blocker_;

  // The number of blockers.

  int blockers_;

  // The single writer.

  const Task* writer_;

  // The list of Tasks waiting for this token to be released.

  Task_list waiting_;

};

// In order to support tokens more reliably, we provide objects which

// handle them using RAII.

// RAII class to get a write lock on a token.  This requires

// specifying the task which is doing the lock.

class Task_write_token

{

 public:

  Task_write_token(Task_token* token, const Task* task)

    : token_(token), task_(task)

  { this->token_->add_writer(this->task_); }

  ~Task_write_token()

  { this->token_->remove_writer(this->task_); }

 private:

  Task_write_token(const Task_write_token&);

  Task_write_token& operator=(const Task_write_token&);

  Task_token* token_;

  const Task* task_;

};

// RAII class for a blocker.

class Task_block_token

{

 public:

  // The blocker count must be incremented when the task is created.

  // This object is created when the task is run, so we don't do

  // anything in the constructor.

  Task_block_token(Task_token* token)

    : token_(token)

  { gold_assert(this->token_->is_blocked()); }

  ~Task_block_token()

  { this->token_->remove_blocker(); }

 private:

  Task_block_token(const Task_block_token&);

  Task_block_token& operator=(const Task_block_token&);

  Task_token* token_;

};

// An object which implements an RAII lock for any object which

// supports lock and unlock methods.

template<typename Obj>

class Task_lock_obj

{

 public:

  Task_lock_obj(const Task* task, Obj* obj)

    : task_(task), obj_(obj)

  { this->obj_->lock(task); }

  ~Task_lock_obj()

  { this->obj_->unlock(this->task_); }

 private:

  Task_lock_obj(const Task_lock_obj&);

  Task_lock_obj& operator=(const Task_lock_obj&);

  const Task* task_;

  Obj* obj_;

};

// A class which holds the set of Task_tokens which must be locked for

// a Task.  No Task requires more than four Task_tokens, so we set

// that as a limit.

class Task_locker

{

 public:

  static const int max_task_count = 4;

  Task_locker()

    : count_(0)

  { }

  ~Task_locker()

  { }

  // Clear the locker.

  void

  clear()

  { this->count_ = 0; }

  // Add a token to the locker.

  void

  add(Task* t, Task_token* token)

  {

    gold_assert(this->count_ < max_task_count);

    this->tokens_[this->count_] = token;

    ++this->count_;

    // A blocker will have been incremented when the task is created.

    // A writer we need to lock now.

    if (!token->is_blocker())

      token->add_writer(t);

  }

  // Iterate over the tokens.

  typedef Task_token** iterator;

  iterator

  begin()

  { return &this->tokens_[0]; }

  iterator

  end()

  { return &this->tokens_[this->count_]; }

 private:

  Task_locker(const Task_locker&);

  Task_locker& operator=(const Task_locker&);

  // The number of tokens.

  int count_;

  // The tokens.

  Task_token* tokens_[max_task_count];

};

} // End namespace gold.

#endif // !defined(GOLD_TOKEN_H)