tor/src/common/workqueue.c

/* Copyright (c) 2013, The Tor Project, Inc. */
/* See LICENSE for licensing information */

#include "orconfig.h"
#include "compat.h"
#include "compat_threads.h"
#include "util.h"
#include "workqueue.h"
#include "tor_queue.h"
#include "torlog.h"

struct threadpool_s {
  /** An array of pointers to workerthread_t: one for each running worker
   * thread. */
  struct workerthread_s **threads;
  /** Index of the next thread that we'll give work to.*/
  int next_for_work;

  /** Number of elements in threads. */
  int n_threads;
  /** Mutex to protect all the above fields. */
  tor_mutex_t lock;

  /** A reply queue to use when constructing new threads. */
  replyqueue_t *reply_queue;

  /** Functions used to allocate and free thread state. */
  void *(*new_thread_state_fn)(void*);
  void (*free_thread_state_fn)(void*);
  void *new_thread_state_arg;
};

struct workqueue_entry_s {
  /** The next workqueue_entry_t that's pending on the same thread or
   * reply queue. */
  TOR_TAILQ_ENTRY(workqueue_entry_s) next_work;
  /** The thread to which this workqueue_entry_t was assigned. This field
   * is set when the workqueue_entry_t is created, and won't be cleared until
   * after it's handled in the main thread. */
  struct workerthread_s *on_thread;
  /** True iff this entry is waiting for a worker to start processing it. */
  uint8_t pending;
  /** Function to run in the worker thread. */
  int (*fn)(void *state, void *arg);
  /** Function to run while processing the reply queue. */
  void (*reply_fn)(void *arg);
  /** Argument for the above functions. */
  void *arg;
};

struct replyqueue_s {
  /** Mutex to protect the answers field */
  tor_mutex_t lock;
  /** Doubly-linked list of answers that the reply queue needs to handle. */
  TOR_TAILQ_HEAD(, workqueue_entry_s) answers;

  /** Mechanism to wake up the main thread when it is receiving answers. */
  alert_sockets_t alert;
};

/** A worker thread represents a single thread in a thread pool.  To avoid
 * contention, each gets its own queue. This breaks the guarantee that that
 * queued work will get executed strictly in order. */
typedef struct workerthread_s {
  /** Lock to protect all fields of this thread and its queue. */
  tor_mutex_t lock;
  /** Condition variable that we wait on when we have no work, and which
   * gets signaled when our queue becomes nonempty. */
  tor_cond_t condition;
  /** Queue of pending work that we have to do. */
  TOR_TAILQ_HEAD(, workqueue_entry_s) work;
  /** True iff this thread is currently in its loop. */
  unsigned is_running;
  /** True iff this thread has crashed or is shut down for some reason. */
  unsigned is_shut_down;
  /** True if we're waiting for more elements to get added to the queue. */
  unsigned waiting;
  /** User-supplied state field that we pass to the worker functions of each
   * work item. */
  void *state;
  /** Reply queue to which we pass our results. */
  replyqueue_t *reply_queue;
} workerthread_t;

static void queue_reply(replyqueue_t *queue, workqueue_entry_t *work);

/** Allocate and return a new workqueue_entry_t, set up to run the function
 * <b>fn</b> in the worker thread, and <b>reply_fn</b> in the main
 * thread. See threadpool_queue_work() for full documentation. */
static workqueue_entry_t *
workqueue_entry_new(int (*fn)(void*, void*),
                    void (*reply_fn)(void*),
                    void *arg)
{
  workqueue_entry_t *ent = tor_malloc_zero(sizeof(workqueue_entry_t));
  ent->fn = fn;
  ent->reply_fn = reply_fn;
  ent->arg = arg;
  return ent;
}

/**
 * Release all storage held in <b>ent</b>. Call only when <b>ent</b> is not on
 * any queue.
 */
static void
workqueue_entry_free(workqueue_entry_t *ent)
{
  if (!ent)
    return;
  tor_free(ent);
}

/**
 * Cancel a workqueue_entry_t that has been returned from
 * threadpool_queue_work.
 *
 * You must not call this function on any work whose reply function has been
 * executed in the main thread; that will cause undefined behavior (probably,
 * a crash).
 *
 * If the work is cancelled, this function return the argument passed to the
 * work function. It is the caller's responsibility to free this storage.
 *
 * This function will have no effect if the worker thread has already executed
 * or begun to execute the work item.  In that case, it will return NULL.
 */
void *
workqueue_entry_cancel(workqueue_entry_t *ent)
{
  int cancelled = 0;
  void *result = NULL;
  tor_mutex_acquire(&ent->on_thread->lock);
  if (ent->pending) {
    TOR_TAILQ_REMOVE(&ent->on_thread->work, ent, next_work);
    cancelled = 1;
    result = ent->arg;
  }
  tor_mutex_release(&ent->on_thread->lock);

  if (cancelled) {
    tor_free(ent);
  }
  return result;
}

/**
 * Main function for the worker thread.
 */
static void
worker_thread_main(void *thread_)
{
  workerthread_t *thread = thread_;
  workqueue_entry_t *work;
  int result;

  tor_mutex_acquire(&thread->lock);
  thread->is_running = 1;
  while (1) {
    /* lock must be held at this point. */
    while (!TOR_TAILQ_EMPTY(&thread->work)) {
      /* lock must be held at this point. */

      work = TOR_TAILQ_FIRST(&thread->work);
      TOR_TAILQ_REMOVE(&thread->work, work, next_work);
      work->pending = 0;
      tor_mutex_release(&thread->lock);

      /* We run the work function without holding the thread lock. This
       * is the main thread's first opportunity to give us more work. */
      result = work->fn(thread->state, work->arg);

      /* Queue the reply for the main thread. */
      queue_reply(thread->reply_queue, work);

      tor_mutex_acquire(&thread->lock);
      /* We may need to exit the thread. */
      if (result >= WQ_RPL_ERROR) {
        thread->is_running = 0;
        thread->is_shut_down = 1;
        tor_mutex_release(&thread->lock);
        return;
      }
    }
    /* At this point the lock is held, and there is no work in this thread's
     * queue. */

    /* TODO: Try work-stealing. */
    /* TODO: support an idle-function */

    /* Okay. Now, wait till somebody has work for us. */
    thread->waiting = 1;
    if (tor_cond_wait(&thread->condition, &thread->lock, NULL) < 0) {
      /* XXXX ERROR */
    }
    thread->waiting = 0;
  }
}

/** Put a reply on the reply queue.  The reply must not currently be on
 * any thread's work queue. */
static void
queue_reply(replyqueue_t *queue, workqueue_entry_t *work)
{
  int was_empty;
  tor_mutex_acquire(&queue->lock);
  was_empty = TOR_TAILQ_EMPTY(&queue->answers);
  TOR_TAILQ_INSERT_TAIL(&queue->answers, work, next_work);
  tor_mutex_release(&queue->lock);

  if (was_empty) {
    if (queue->alert.alert_fn(queue->alert.write_fd) < 0) {
      /* XXXX complain! */
    }
  }
}

/** Allocate and start a new worker thread to use state object <b>state</b>,
 * and send responses to <b>replyqueue</b>. */
static workerthread_t *
workerthread_new(void *state, replyqueue_t *replyqueue)
{
  workerthread_t *thr = tor_malloc_zero(sizeof(workerthread_t));
  tor_mutex_init_for_cond(&thr->lock);
  tor_cond_init(&thr->condition);
  TOR_TAILQ_INIT(&thr->work);
  thr->state = state;
  thr->reply_queue = replyqueue;

  if (spawn_func(worker_thread_main, thr) < 0) {
    log_err(LD_GENERAL, "Can't launch worker thread.");
    return NULL;
  }

  return thr;
}

/**
 * Add an item of work to a single worker thread. See threadpool_queue_work(*)
 * for arguments.
 */
static workqueue_entry_t *
workerthread_queue_work(workerthread_t *worker,
                        int (*fn)(void *, void *),
                        void (*reply_fn)(void *),
                        void *arg)
{
  workqueue_entry_t *ent = workqueue_entry_new(fn, reply_fn, arg);

  tor_mutex_acquire(&worker->lock);
  ent->on_thread = worker;
  ent->pending = 1;
  TOR_TAILQ_INSERT_TAIL(&worker->work, ent, next_work);

  if (worker->waiting) /* XXXX inside or outside of lock?? */
    tor_cond_signal_one(&worker->condition);

  tor_mutex_release(&worker->lock);
  return ent;
}

/**
 * Queue an item of work for a thread in a thread pool.  The function
 * <b>fn</b> will be run in a worker thread, and will receive as arguments the
 * thread's state object, and the provided object <b>arg</b>. It must return
 * one of WQ_RPL_REPLY, WQ_RPL_ERROR, or WQ_RPL_SHUTDOWN.
 *
 * Regardless of its return value, the function <b>reply_fn</b> will later be
 * run in the main thread when it invokes replyqueue_process(), and will
 * receive as its argument the same <b>arg</b> object.  It's the reply
 * function's responsibility to free the work object.
 *
 * On success, return a workqueue_entry_t object that can be passed to
 * workqueue_entry_cancel(). On failure, return NULL.
 *
 * Note that because each thread has its own work queue, work items may not
 * be executed strictly in order.
 */
workqueue_entry_t *
threadpool_queue_work(threadpool_t *pool,
                      int (*fn)(void *, void *),
                      void (*reply_fn)(void *),
                      void *arg)
{
  workerthread_t *worker;

  tor_mutex_acquire(&pool->lock);
  /* Pick the next thread in random-access order. */
  worker = pool->threads[pool->next_for_work++];
  if (!worker) {
    tor_mutex_release(&pool->lock);
    return NULL;
  }
  if (pool->next_for_work >= pool->n_threads)
    pool->next_for_work = 0;
  tor_mutex_release(&pool->lock);

  return workerthread_queue_work(worker, fn, reply_fn, arg);
}

/**
 * Queue a copy of a work item for every thread in a pool.  This can be used,
 * for example, to tell the threads to update some parameter in their states.
 *
 * Arguments are as for <b>threadpool_queue_work</b>, except that the
 * <b>arg</b> value is passed to <b>dup_fn</b> once per each thread to
 * make a copy of it.
 *
 * Return 0 on success, -1 on failure.
 */
int
threadpool_queue_for_all(threadpool_t *pool,
                         void *(*dup_fn)(const void *),
                         int (*fn)(void *, void *),
                         void (*reply_fn)(void *),
                         void *arg)
{
  int i = 0;
  workerthread_t *worker;
  void *arg_copy;
  while (1) {
    tor_mutex_acquire(&pool->lock);
    if (i >= pool->n_threads) {
      tor_mutex_release(&pool->lock);
      return 0;
    }
    worker = pool->threads[i++];
    tor_mutex_release(&pool->lock);

    arg_copy = dup_fn ? dup_fn(arg) : arg;
    /* CHECK*/ workerthread_queue_work(worker, fn, reply_fn, arg_copy);
  }
}

/** Launch threads until we have <b>n</b>. */
static int
threadpool_start_threads(threadpool_t *pool, int n)
{
  tor_mutex_acquire(&pool->lock);

  if (pool->n_threads < n)
    pool->threads = tor_realloc(pool->threads, sizeof(workerthread_t*)*n);

  while (pool->n_threads < n) {
    void *state = pool->new_thread_state_fn(pool->new_thread_state_arg);
    workerthread_t *thr = workerthread_new(state, pool->reply_queue);

    if (!thr) {
      tor_mutex_release(&pool->lock);
      return -1;
    }
    pool->threads[pool->n_threads++] = thr;
  }
  tor_mutex_release(&pool->lock);

  return 0;
}

/**
 * Construct a new thread pool with <b>n</b> worker threads, configured to
 * send their output to <b>replyqueue</b>.  The threads' states will be
 * constructed with the <b>new_thread_state_fn</b> call, receiving <b>arg</b>
 * as its argument.  When the threads close, they will call
 * <b>free_thread_state_fn</b> on their states.
 */
threadpool_t *
threadpool_new(int n_threads,
               replyqueue_t *replyqueue,
               void *(*new_thread_state_fn)(void*),
               void (*free_thread_state_fn)(void*),
               void *arg)
{
  threadpool_t *pool;
  pool = tor_malloc_zero(sizeof(threadpool_t));
  tor_mutex_init(&pool->lock);
  pool->new_thread_state_fn = new_thread_state_fn;
  pool->new_thread_state_arg = arg;
  pool->free_thread_state_fn = free_thread_state_fn;
  pool->reply_queue = replyqueue;

  if (threadpool_start_threads(pool, n_threads) < 0) {
    tor_mutex_uninit(&pool->lock);
    tor_free(pool);
    return NULL;
  }

  return pool;
}

/** Return the reply queue associated with a given thread pool. */
replyqueue_t *
threadpool_get_replyqueue(threadpool_t *tp)
{
  return tp->reply_queue;
}

/** Allocate a new reply queue.  Reply queues are used to pass results from
 * worker threads to the main thread.  Since the main thread is running an
 * IO-centric event loop, it needs to get woken up with means other than a
 * condition variable. */
replyqueue_t *
replyqueue_new(uint32_t alertsocks_flags)
{
  replyqueue_t *rq;

  rq = tor_malloc_zero(sizeof(replyqueue_t));
  if (alert_sockets_create(&rq->alert, alertsocks_flags) < 0) {
    tor_free(rq);
    return NULL;
  }

  tor_mutex_init(&rq->lock);
  TOR_TAILQ_INIT(&rq->answers);

  return rq;
}

/**
 * Return the "read socket" for a given reply queue.  The main thread should
 * listen for read events on this socket, and call replyqueue_process() every
 * time it triggers.
 */
tor_socket_t
replyqueue_get_socket(replyqueue_t *rq)
{
  return rq->alert.read_fd;
}

/**
 * Process all pending replies on a reply queue. The main thread should call
 * this function every time the socket returned by replyqueue_get_socket() is
 * readable.
 */
void
replyqueue_process(replyqueue_t *queue)
{
  if (queue->alert.drain_fn(queue->alert.read_fd) < 0) {
    /* XXXX complain! */
  }

  tor_mutex_acquire(&queue->lock);
  while (!TOR_TAILQ_EMPTY(&queue->answers)) {
    /* lock must be held at this point.*/
    workqueue_entry_t *work = TOR_TAILQ_FIRST(&queue->answers);
    TOR_TAILQ_REMOVE(&queue->answers, work, next_work);
    tor_mutex_release(&queue->lock);

    work->reply_fn(work->arg);
    workqueue_entry_free(work);

    tor_mutex_acquire(&queue->lock);
  }

  tor_mutex_release(&queue->lock);
}