kern/condition: new module

1 files changed, 164 insertions, 0 deletions

diff --git a/kern/condition.c b/kern/condition.c
new file mode 100644
index 00000000..6f7bec4c
--- /dev/null
+++ b/kern/condition.c
@@ -0,0 +1,164 @@
+/*
+ * Copyright (c) 2013 Richard Braun.
+ *
+ * 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, see <http://www.gnu.org/licenses/>.
+ *
+ *
+ * In order to avoid the infamous thundering herd problem, this implementation
+ * doesn't wake all threads when broadcasting a condition. Instead, they are
+ * queued on the mutex associated with the condition, and an attempt to wake
+ * one by locking and unlocking the mutex is performed. If the mutex is already
+ * locked, the current owner does the same when unlocking.
+ *
+ * TODO Refactor mutex and condition code.
+ */
+
+#include <kern/assert.h>
+#include <kern/condition.h>
+#include <kern/list.h>
+#include <kern/mutex.h>
+#include <kern/spinlock.h>
+#include <kern/stddef.h>
+#include <kern/thread.h>
+#include <machine/atomic.h>
+
+void
+condition_init(struct condition *condition)
+{
+    spinlock_init(&condition->lock);
+    condition->mutex = NULL;
+    list_init(&condition->waiters);
+}
+
+void
+condition_wait(struct condition *condition, struct mutex *mutex)
+{
+    struct mutex_waiter waiter, *waiter_ptr;
+    unsigned long state;
+
+    waiter.thread = thread_self();
+
+    spinlock_lock(&condition->lock);
+
+    assert((condition->mutex == NULL) || (condition->mutex == mutex));
+
+    if (condition->mutex == NULL)
+        condition->mutex = mutex;
+
+    list_insert_tail(&condition->waiters, &waiter.node);
+
+    spinlock_lock(&mutex->lock);
+
+    state = atomic_swap(&mutex->state, MUTEX_UNLOCKED);
+
+    if (state != MUTEX_LOCKED) {
+        assert(state == MUTEX_CONTENDED);
+
+        if (!list_empty(&mutex->waiters)) {
+            waiter_ptr = list_first_entry(&mutex->waiters, struct mutex_waiter,
+                                          node);
+            thread_wakeup(waiter_ptr->thread);
+        }
+    }
+
+    spinlock_unlock(&condition->lock);
+
+    do {
+        thread_sleep(&mutex->lock);
+        state = atomic_swap(&mutex->state, MUTEX_CONTENDED);
+    } while (state != MUTEX_UNLOCKED);
+
+    list_remove(&waiter.node);
+
+    if (list_empty(&mutex->waiters)) {
+        state = atomic_swap(&mutex->state, MUTEX_LOCKED);
+        assert(state == MUTEX_CONTENDED);
+    }
+
+    spinlock_unlock(&mutex->lock);
+}
+
+void
+condition_signal(struct condition *condition)
+{
+    struct mutex_waiter *waiter;
+    struct mutex *mutex;
+    unsigned long state;
+
+    spinlock_lock(&condition->lock);
+
+    if (condition->mutex == NULL) {
+        spinlock_unlock(&condition->lock);
+        return;
+    }
+
+    mutex = condition->mutex;
+    waiter = list_first_entry(&condition->waiters, struct mutex_waiter, node);
+    list_remove(&waiter->node);
+
+    if (list_empty(&condition->waiters))
+        condition->mutex = NULL;
+
+    spinlock_unlock(&condition->lock);
+
+    spinlock_lock(&mutex->lock);
+
+    list_insert_tail(&mutex->waiters, &waiter->node);
+    state = atomic_swap(&mutex->state, MUTEX_CONTENDED);
+
+    if (state == MUTEX_UNLOCKED) {
+        state = atomic_swap(&mutex->state, MUTEX_UNLOCKED);
+        assert(state == MUTEX_CONTENDED);
+        thread_wakeup(waiter->thread);
+    }
+
+    spinlock_unlock(&mutex->lock);
+}
+
+void
+condition_broadcast(struct condition *condition)
+{
+    struct mutex_waiter *waiter;
+    struct mutex *mutex;
+    struct list tmp;
+    unsigned long state;
+
+    spinlock_lock(&condition->lock);
+
+    if (condition->mutex == NULL) {
+        spinlock_unlock(&condition->lock);
+        return;
+    }
+
+    mutex = condition->mutex;
+    condition->mutex = NULL;
+    list_set_head(&tmp, &condition->waiters);
+    list_init(&condition->waiters);
+
+    spinlock_unlock(&condition->lock);
+
+    spinlock_lock(&mutex->lock);
+
+    list_concat(&mutex->waiters, &tmp);
+    state = atomic_swap(&mutex->state, MUTEX_CONTENDED);
+
+    if (state == MUTEX_UNLOCKED) {
+        state = atomic_swap(&mutex->state, MUTEX_UNLOCKED);
+        assert(state == MUTEX_CONTENDED);
+        waiter = list_first_entry(&mutex->waiters, struct mutex_waiter, node);
+        thread_wakeup(waiter->thread);
+    }
+
+    spinlock_unlock(&mutex->lock);
+}