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RIOT/sys/posix/pthread/pthread_barrier.c 3.24 KB
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  /*
   * POSIX compatible implementation of barriers.
   *
   * Copyright (C) 2014 Freie Universität Berlin
   *
   * This file is subject to the terms and conditions of the GNU Lesser
   * General Public License v2.1. See the file LICENSE in the top level
   * directory for more details.
   */
  
  /**
   * @ingroup pthread
   * @{
   * @file
   * @brief Synchronization barriers.
   * @author René Kijewski <kijewski@inf.fu-berlin.de>
   * @}
   */
  
  #include "sched.h"
  #include "pthread.h"
  
  #define ENABLE_DEBUG (0)
  #include "debug.h"
  
  int pthread_barrier_init(pthread_barrier_t *barrier,
                           const pthread_barrierattr_t *attr,
                           unsigned int count)
  {
      (void) attr;
      barrier->next = NULL;
      mutex_init(&barrier->mutex);
      barrier->count = count;
      return 0;
  }
  
  int pthread_barrier_destroy(pthread_barrier_t *barrier)
  {
      barrier->count = -1;
      return 0;
  }
  
  static inline int priority_min(int a, int b)
  {
      if (a == -1) {
          return b;
      }
      return a < b ? a : b;
  }
  
  int pthread_barrier_wait(pthread_barrier_t *barrier)
  {
      /* Idea: the count is decreased by every thread that waits on the barrier.
       * If the value is bigger than zero afterwards, then the thread has to wait
       * to be woken up. Once the value reaches zero, everyone gets woken up. */
  
      mutex_lock(&barrier->mutex);
      DEBUG("%s: hit a synchronization barrier. pid=%" PRIkernel_pid"\n",
            sched_active_thread->name, sched_active_pid);
  
      int switch_prio = -1;
  
      if (--barrier->count > 0) {
          /* need to wait for further threads */
  
          DEBUG("%s: waiting for %u threads. pid=%" PRIkernel_pid "\n",
                sched_active_thread->name, barrier->count, sched_active_pid);
  
          pthread_barrier_waiting_node_t node;
          node.pid = sched_active_pid;
          node.next = barrier->next;
          node.cont = 0;
  
          barrier->next = &node;
          mutex_unlock(&barrier->mutex);
  
          while (1) {
              /* The mutex is reacquired before checking if we should continue,
               * so that the waiting thread don't accidentially run before the
               * wake up loop has ended. Otherwise the thread could run into the
               * the barrier again before `barrier->count` was reset. */
              mutex_lock(&barrier->mutex);
              if (node.cont) {
                  break;
              }
              mutex_unlock_and_sleep(&barrier->mutex);
          }
      }
      else {
          /* all threads have arrived, wake everybody up */
  
          DEBUG("%s: waking every other thread up. pid=%" PRIkernel_pid "\n",
                sched_active_thread->name, sched_active_pid);
  
          int count = 1; /* Count number of woken up threads.
                          * The first thread is the current thread. */
          pthread_barrier_waiting_node_t *next;
          for (next = barrier->next; next; next = next->next) {
              ++count;
              next->cont = 1;
  
              thread_t *other = (thread_t *) sched_threads[next->pid];
              switch_prio = priority_min(switch_prio, other->priority);
              sched_set_status(other, STATUS_PENDING);
          }
          barrier->next = NULL;
          barrier->count = count;
      }
  
      mutex_unlock(&barrier->mutex);
  
      if (switch_prio != -1) {
          sched_switch(switch_prio);
      }
  
      return 0;
  }