static void mspin_unlock(struct mspin_node **lock, struct mspin_node *node)
{
struct mspin_node *next = ACCESS_ONCE(node->next);
if (likely(!next)) {
/*
* Release the lock by setting it to NULL
*/
if (cmpxchg(lock, node, NULL) == node)
return;
/* Wait until the next pointer is set */
while (!(next = ACCESS_ONCE(node->next)))
arch_mutex_cpu_relax();
}
ACCESS_ONCE(next->locked) = 1;
smp_wmb();
}
/*
* Look out! "owner" is an entirely speculative pointer
* access and not reliable.
*/
static noinline
int mutex_spin_on_owner(struct mutex *lock, struct task_struct *owner)
{
rcu_read_lock();
while (owner_running(lock, owner)) {
if (need_resched())
break;
arch_mutex_cpu_relax();
}
rcu_read_unlock();
/*
* We break out the loop above on need_resched() and when the
* owner changed, which is a sign for heavy contention. Return
* success only when lock->owner is NULL.
*/
return lock->owner == NULL;
}
static noinline void mspin_lock(mspin_lock_t *lock, mspin_node_t *node)
{
mspin_node_t *prev;
/* Init node */
node->locked = 0;
node->next = NULL;
prev = xchg(lock, node);
if (likely(prev == NULL)) {
/* Lock acquired */
node->locked = 1;
return;
}
ACCESS_ONCE(prev->next) = node;
smp_wmb();
/* Wait until the lock holder passes the lock down */
while (!ACCESS_ONCE(node->locked))
arch_mutex_cpu_relax();
}
/*
* Lock a mutex (possibly interruptible), slowpath:
*/
static inline int __sched
__mutex_lock_common(struct mutex *lock, long state, unsigned int subclass,
struct lockdep_map *nest_lock, unsigned long ip)
{
struct task_struct *task = current;
struct mutex_waiter waiter;
unsigned long flags;
preempt_disable();
mutex_acquire_nest(&lock->dep_map, subclass, 0, nest_lock, ip);
#ifdef CONFIG_MUTEX_SPIN_ON_OWNER
/*
* Optimistic spinning.
*
* We try to spin for acquisition when we find that there are no
* pending waiters and the lock owner is currently running on a
* (different) CPU.
*
* The rationale is that if the lock owner is running, it is likely to
* release the lock soon.
*
* Since this needs the lock owner, and this mutex implementation
* doesn't track the owner atomically in the lock field, we need to
* track it non-atomically.
*
* We can't do this for DEBUG_MUTEXES because that relies on wait_lock
* to serialize everything.
*
* The mutex spinners are queued up using MCS lock so that only one
* spinner can compete for the mutex. However, if mutex spinning isn't
* going to happen, there is no point in going through the lock/unlock
* overhead.
*/
if (!mutex_can_spin_on_owner(lock))
goto slowpath;
for (;;) {
struct task_struct *owner;
struct mspin_node node;
/*
* If there's an owner, wait for it to either
* release the lock or go to sleep.
*/
mspin_lock(MLOCK(lock), &node);
owner = ACCESS_ONCE(lock->owner);
if (owner && !mutex_spin_on_owner(lock, owner)) {
mspin_unlock(MLOCK(lock), &node);
break;
}
if ((atomic_read(&lock->count) == 1) &&
(atomic_cmpxchg(&lock->count, 1, 0) == 1)) {
lock_acquired(&lock->dep_map, ip);
mutex_set_owner(lock);
mspin_unlock(MLOCK(lock), &node);
preempt_enable();
return 0;
}
mspin_unlock(MLOCK(lock), &node);
/*
* When there's no owner, we might have preempted between the
* owner acquiring the lock and setting the owner field. If
* we're an RT task that will live-lock because we won't let
* the owner complete.
*/
if (!owner && (need_resched() || rt_task(task)))
break;
/*
* The cpu_relax() call is a compiler barrier which forces
* everything in this loop to be re-loaded. We don't need
* memory barriers as we'll eventually observe the right
* values at the cost of a few extra spins.
*/
arch_mutex_cpu_relax();
}
slowpath:
#endif
spin_lock_mutex(&lock->wait_lock, flags);
debug_mutex_lock_common(lock, &waiter);
debug_mutex_add_waiter(lock, &waiter, task_thread_info(task));
/* add waiting tasks to the end of the waitqueue (FIFO): */
list_add_tail(&waiter.list, &lock->wait_list);
waiter.task = task;
if (MUTEX_SHOW_NO_WAITER(lock) && (atomic_xchg(&lock->count, -1) == 1))
goto done;
lock_contended(&lock->dep_map, ip);
for (;;) {
/*
* Lets try to take the lock again - this is needed even if
* we get here for the first time (shortly after failing to
* acquire the lock), to make sure that we get a wakeup once
* it's unlocked. Later on, if we sleep, this is the
* operation that gives us the lock. We xchg it to -1, so
* that when we release the lock, we properly wake up the
* other waiters:
*/
if (MUTEX_SHOW_NO_WAITER(lock) &&
(atomic_xchg(&lock->count, -1) == 1))
break;
/*
* got a signal? (This code gets eliminated in the
* TASK_UNINTERRUPTIBLE case.)
*/
if (unlikely(signal_pending_state(state, task))) {
mutex_remove_waiter(lock, &waiter,
task_thread_info(task));
mutex_release(&lock->dep_map, 1, ip);
spin_unlock_mutex(&lock->wait_lock, flags);
debug_mutex_free_waiter(&waiter);
preempt_enable();
return -EINTR;
}
__set_task_state(task, state);
/* didn't get the lock, go to sleep: */
spin_unlock_mutex(&lock->wait_lock, flags);
schedule_preempt_disabled();
spin_lock_mutex(&lock->wait_lock, flags);
}
done:
lock_acquired(&lock->dep_map, ip);
/* got the lock - rejoice! */
mutex_remove_waiter(lock, &waiter, current_thread_info());
mutex_set_owner(lock);
/* set it to 0 if there are no waiters left: */
if (likely(list_empty(&lock->wait_list)))
atomic_set(&lock->count, 0);
spin_unlock_mutex(&lock->wait_lock, flags);
debug_mutex_free_waiter(&waiter);
preempt_enable();
return 0;
}