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/*
* 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>
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);
}
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