Add pthread_barrier_* functions

Compare [`pthread_barrier_init`][1].

  [1]: http://pubs.opengroup.org/onlinepubs/9699919799/functions/pthread_barrier_init.html

sys/posix/pthread/include/pthreadtypes.h

@@ -19,8 +19,24 @@ typedef int pthread_once_t;
 /* Single execution handling.  */
 #define PTHREAD_ONCE_INIT 0
 
-typedef unsigned long int pthread_barrier_t;
-typedef unsigned long int pthread_barrierattr_t;
+typedef struct pthread_barrier_waiting_node
+{
+    struct pthread_barrier_waiting_node *next;
+    int pid;
+    volatile int cont;
+} pthread_barrier_waiting_node_t;
+
+typedef struct pthread_barrier
+{
+    struct pthread_barrier_waiting_node *next;
+    mutex_t mutex;
+    volatile int count;
+} pthread_barrier_t;
+
+typedef struct pthread_barrierattr
+{
+    int pshared;
+} pthread_barrierattr_t;
 
 typedef unsigned long int pthread_cond_t;
 typedef unsigned long int pthread_condattr_t;

sys/posix/pthread/pthread_barrier.c

@@ -0,0 +1,98 @@
+/**
+* POSIX compatible implementation of barriers.
+*
+* Copyright (C) 2014 Freie Universität Berlin
+*
+* This file subject to the terms and conditions of the GNU Lesser General
+* Public License. See the file LICENSE in the top level directory for more
+* details.
+*
+* @ingroup posix
+* @{
+* @file
+* @brief Implementation of the pthread barrier.
+* @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;
+}
+
+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=%u\n",
+          active_thread->name, active_thread->pid);
+
+    if (--barrier->count > 0) {
+        /* need to wait for further threads */
+
+        DEBUG("%s: waiting for %u threads. pid=%u\n",
+              active_thread->name, barrier->count, active_thread->pid);
+
+        pthread_barrier_waiting_node_t node;
+        node.pid = thread_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=%u\n",
+              active_thread->name, active_thread->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;
+            sched_set_status((tcb_t *) sched_threads[next->pid], STATUS_PENDING);
+        }
+        barrier->next = NULL;
+        barrier->count = count;
+    }
+
+    mutex_unlock(&barrier->mutex);
+    return 0;
+}

sys/posix/pthread/pthread_barrierattr.c

@@ -0,0 +1,44 @@
+/**
+* POSIX compatible implementation of barriers.
+*
+* Copyright (C) 2014 Freie Universität Berlin
+*
+* This file subject to the terms and conditions of the GNU Lesser General
+* Public License. See the file LICENSE in the top level directory for more
+* details.
+*
+* @ingroup posix
+* @{
+* @file
+* @brief Implementation of the pthread barrier attributes.
+* @author René Kijewski <kijewski@inf.fu-berlin.de>
+* @}
+*/
+
+#include "pthread.h"
+
+#include <stdbool.h>
+
+int pthread_barrierattr_init(pthread_barrierattr_t *attr)
+{
+    attr->pshared = 0;
+    return 0;
+}
+
+int pthread_barrierattr_destroy(pthread_barrierattr_t *attr)
+{
+    (void) attr;
+    return 0;
+}
+
+int pthread_barrierattr_getpshared(const pthread_barrierattr_t *attr, int *pshared)
+{
+    *pshared = attr->pshared;
+    return 0;
+}
+
+int pthread_barrierattr_setpshared(pthread_barrierattr_t *attr, int pshared)
+{
+    attr->pshared = pshared;
+    return 0;
+}