kern/mutex: new module

As the name implies, this module provides sleepable mutual exclusion locks.

1 files changed, 120 insertions, 0 deletions

diff --git a/kern/mutex.c b/kern/mutex.c
new file mode 100644
index 00000000..507116e2
--- /dev/null
+++ b/kern/mutex.c
@@ -0,0 +1,120 @@
+/*
+ * 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/>.
+ */
+
+#include <kern/assert.h>
+#include <kern/list.h>
+#include <kern/mutex.h>
+#include <kern/spinlock.h>
+#include <kern/thread.h>
+#include <machine/atomic.h>
+
+struct mutex_waiter {
+    struct list node;
+    struct thread *thread;
+};
+
+void
+mutex_init(struct mutex *mutex)
+{
+    mutex->state = MUTEX_UNLOCKED;
+    spinlock_init(&mutex->lock);
+    list_init(&mutex->waiters);
+}
+
+int
+mutex_trylock(struct mutex *mutex)
+{
+    unsigned long state;
+
+    state = atomic_cas(&mutex->state, MUTEX_UNLOCKED, MUTEX_LOCKED);
+
+    if (state == MUTEX_UNLOCKED)
+        return 0;
+
+    return 1;
+}
+
+void
+mutex_lock(struct mutex *mutex)
+{
+    struct mutex_waiter waiter;
+    unsigned long state;
+
+    state = atomic_cas(&mutex->state, MUTEX_UNLOCKED, MUTEX_LOCKED);
+
+    if (state == MUTEX_UNLOCKED)
+        return;
+
+    /*
+     * The mutex was either locked or contended. Unconditionnally update its
+     * state to reflect it is now contended, and to check the previous state
+     * while holding the waiters lock so that the current thread doesn't miss
+     * a wakeup when the owner unlocks.
+     */
+
+    assert((state == MUTEX_LOCKED) || (state == MUTEX_CONTENDED));
+
+    spinlock_lock(&mutex->lock);
+
+    state = atomic_swap(&mutex->state, MUTEX_CONTENDED);
+
+    if (state == MUTEX_UNLOCKED)
+        goto out;
+
+    waiter.thread = thread_self();
+    list_insert_tail(&mutex->waiters, &waiter.node);
+
+    do {
+        thread_sleep(&mutex->lock);
+        state = atomic_swap(&mutex->state, MUTEX_CONTENDED);
+    } while (state != MUTEX_UNLOCKED);
+
+    list_remove(&waiter.node);
+
+out:
+    if (list_empty(&mutex->waiters)) {
+        state = atomic_swap(&mutex->state, MUTEX_LOCKED);
+        assert(state == MUTEX_CONTENDED);
+    }
+
+    spinlock_unlock(&mutex->lock);
+}
+
+void
+mutex_unlock(struct mutex *mutex)
+{
+    struct mutex_waiter *waiter;
+    unsigned long state;
+
+    state = atomic_swap(&mutex->state, MUTEX_UNLOCKED);
+
+    if (state == MUTEX_LOCKED)
+        return;
+
+    /* The mutex was contended, wake up the next waiter if any */
+
+    assert(state == MUTEX_CONTENDED);
+
+    spinlock_lock(&mutex->lock);
+
+    if (!list_empty(&mutex->waiters)) {
+        waiter = list_first_entry(&mutex->waiters, struct mutex_waiter, node);
+        thread_wakeup(waiter->thread);
+    }
+
+    spinlock_unlock(&mutex->lock);
+}