owner = *p;
} while (cmpxchg(p, owner, owner | RT_MUTEX_HAS_WAITERS) != owner);
}
+
+/*
+ * Safe fastpath aware unlock:
+ * 1) Clear the waiters bit
+ * 2) Drop lock->wait_lock
+ * 3) Try to unlock the lock with cmpxchg
+ */
+static inline bool unlock_rt_mutex_safe(struct rt_mutex *lock)
+ __releases(lock->wait_lock)
+{
+ struct task_struct *owner = rt_mutex_owner(lock);
+
+ clear_rt_mutex_waiters(lock);
+ raw_spin_unlock(&lock->wait_lock);
+ /*
+ * If a new waiter comes in between the unlock and the cmpxchg
+ * we have two situations:
+ *
+ * unlock(wait_lock);
+ * lock(wait_lock);
+ * cmpxchg(p, owner, 0) == owner
+ * mark_rt_mutex_waiters(lock);
+ * acquire(lock);
+ * or:
+ *
+ * unlock(wait_lock);
+ * lock(wait_lock);
+ * mark_rt_mutex_waiters(lock);
+ *
+ * cmpxchg(p, owner, 0) != owner
+ * enqueue_waiter();
+ * unlock(wait_lock);
+ * lock(wait_lock);
+ * wake waiter();
+ * unlock(wait_lock);
+ * lock(wait_lock);
+ * acquire(lock);
+ */
+ return rt_mutex_cmpxchg(lock, owner, NULL);
+}
+
#else
# define rt_mutex_cmpxchg(l,c,n) (0)
static inline void mark_rt_mutex_waiters(struct rt_mutex *lock)
lock->owner = (struct task_struct *)
((unsigned long)lock->owner | RT_MUTEX_HAS_WAITERS);
}
+
+/*
+ * Simple slow path only version: lock->owner is protected by lock->wait_lock.
+ */
+static inline bool unlock_rt_mutex_safe(struct rt_mutex *lock)
+ __releases(lock->wait_lock)
+{
+ lock->owner = NULL;
+ raw_spin_unlock(&lock->wait_lock);
+ return true;
+}
#endif
static inline int
*/
int max_lock_depth = 1024;
+static inline struct rt_mutex *task_blocked_on_lock(struct task_struct *p)
+{
+ return p->pi_blocked_on ? p->pi_blocked_on->lock : NULL;
+}
+
/*
* Adjust the priority chain. Also used for deadlock detection.
* Decreases task's usage by one - may thus free the task.
*
- * @task: the task owning the mutex (owner) for which a chain walk is probably
- * needed
+ * @task: the task owning the mutex (owner) for which a chain walk is
+ * probably needed
* @deadlock_detect: do we have to carry out deadlock detection?
- * @orig_lock: the mutex (can be NULL if we are walking the chain to recheck
- * things for a task that has just got its priority adjusted, and
- * is waiting on a mutex)
+ * @orig_lock: the mutex (can be NULL if we are walking the chain to recheck
+ * things for a task that has just got its priority adjusted, and
+ * is waiting on a mutex)
+ * @next_lock: the mutex on which the owner of @orig_lock was blocked before
+ * we dropped its pi_lock. Is never dereferenced, only used for
+ * comparison to detect lock chain changes.
* @orig_waiter: rt_mutex_waiter struct for the task that has just donated
- * its priority to the mutex owner (can be NULL in the case
- * depicted above or if the top waiter is gone away and we are
- * actually deboosting the owner)
- * @top_task: the current top waiter
+ * its priority to the mutex owner (can be NULL in the case
+ * depicted above or if the top waiter is gone away and we are
+ * actually deboosting the owner)
+ * @top_task: the current top waiter
*
* Returns 0 or -EDEADLK.
*/
static int rt_mutex_adjust_prio_chain(struct task_struct *task,
int deadlock_detect,
struct rt_mutex *orig_lock,
+ struct rt_mutex *next_lock,
struct rt_mutex_waiter *orig_waiter,
struct task_struct *top_task)
{
}
put_task_struct(task);
- return deadlock_detect ? -EDEADLK : 0;
+ return -EDEADLK;
}
retry:
/*
if (orig_waiter && !rt_mutex_owner(orig_lock))
goto out_unlock_pi;
+ /*
+ * We dropped all locks after taking a refcount on @task, so
+ * the task might have moved on in the lock chain or even left
+ * the chain completely and blocks now on an unrelated lock or
+ * on @orig_lock.
+ *
+ * We stored the lock on which @task was blocked in @next_lock,
+ * so we can detect the chain change.
+ */
+ if (next_lock != waiter->lock)
+ goto out_unlock_pi;
+
/*
* Drop out, when the task has no waiters. Note,
* top_waiter can be NULL, when we are in the deboosting
if (lock == orig_lock || rt_mutex_owner(lock) == top_task) {
debug_rt_mutex_deadlock(deadlock_detect, orig_waiter, lock);
raw_spin_unlock(&lock->wait_lock);
- ret = deadlock_detect ? -EDEADLK : 0;
+ ret = -EDEADLK;
goto out_unlock_pi;
}
__rt_mutex_adjust_prio(task);
}
+ /*
+ * Check whether the task which owns the current lock is pi
+ * blocked itself. If yes we store a pointer to the lock for
+ * the lock chain change detection above. After we dropped
+ * task->pi_lock next_lock cannot be dereferenced anymore.
+ */
+ next_lock = task_blocked_on_lock(task);
+
raw_spin_unlock_irqrestore(&task->pi_lock, flags);
top_waiter = rt_mutex_top_waiter(lock);
raw_spin_unlock(&lock->wait_lock);
+ /*
+ * We reached the end of the lock chain. Stop right here. No
+ * point to go back just to figure that out.
+ */
+ if (!next_lock)
+ goto out_put_task;
+
if (!detect_deadlock && waiter != top_waiter)
goto out_put_task;
*
* Must be called with lock->wait_lock held.
*
- * @lock: the lock to be acquired.
- * @task: the task which wants to acquire the lock
- * @waiter: the waiter that is queued to the lock's wait list. (could be NULL)
+ * @lock: The lock to be acquired.
+ * @task: The task which wants to acquire the lock
+ * @waiter: The waiter that is queued to the lock's wait list if the
+ * callsite called task_blocked_on_lock(), otherwise NULL
*/
static int try_to_take_rt_mutex(struct rt_mutex *lock, struct task_struct *task,
- struct rt_mutex_waiter *waiter)
+ struct rt_mutex_waiter *waiter)
{
+ unsigned long flags;
+
/*
- * We have to be careful here if the atomic speedups are
- * enabled, such that, when
- * - no other waiter is on the lock
- * - the lock has been released since we did the cmpxchg
- * the lock can be released or taken while we are doing the
- * checks and marking the lock with RT_MUTEX_HAS_WAITERS.
+ * Before testing whether we can acquire @lock, we set the
+ * RT_MUTEX_HAS_WAITERS bit in @lock->owner. This forces all
+ * other tasks which try to modify @lock into the slow path
+ * and they serialize on @lock->wait_lock.
+ *
+ * The RT_MUTEX_HAS_WAITERS bit can have a transitional state
+ * as explained at the top of this file if and only if:
*
- * The atomic acquire/release aware variant of
- * mark_rt_mutex_waiters uses a cmpxchg loop. After setting
- * the WAITERS bit, the atomic release / acquire can not
- * happen anymore and lock->wait_lock protects us from the
- * non-atomic case.
+ * - There is a lock owner. The caller must fixup the
+ * transient state if it does a trylock or leaves the lock
+ * function due to a signal or timeout.
*
- * Note, that this might set lock->owner =
- * RT_MUTEX_HAS_WAITERS in the case the lock is not contended
- * any more. This is fixed up when we take the ownership.
- * This is the transitional state explained at the top of this file.
+ * - @task acquires the lock and there are no other
+ * waiters. This is undone in rt_mutex_set_owner(@task) at
+ * the end of this function.
*/
mark_rt_mutex_waiters(lock);
+ /*
+ * If @lock has an owner, give up.
+ */
if (rt_mutex_owner(lock))
return 0;
/*
- * It will get the lock because of one of these conditions:
- * 1) there is no waiter
- * 2) higher priority than waiters
- * 3) it is top waiter
+ * If @waiter != NULL, @task has already enqueued the waiter
+ * into @lock waiter list. If @waiter == NULL then this is a
+ * trylock attempt.
*/
- if (rt_mutex_has_waiters(lock)) {
- if (task->prio >= rt_mutex_top_waiter(lock)->prio) {
- if (!waiter || waiter != rt_mutex_top_waiter(lock))
- return 0;
- }
- }
-
- if (waiter || rt_mutex_has_waiters(lock)) {
- unsigned long flags;
- struct rt_mutex_waiter *top;
-
- raw_spin_lock_irqsave(&task->pi_lock, flags);
+ if (waiter) {
+ /*
+ * If waiter is not the highest priority waiter of
+ * @lock, give up.
+ */
+ if (waiter != rt_mutex_top_waiter(lock))
+ return 0;
- /* remove the queued waiter. */
- if (waiter) {
- rt_mutex_dequeue(lock, waiter);
- task->pi_blocked_on = NULL;
- }
+ /*
+ * We can acquire the lock. Remove the waiter from the
+ * lock waiters list.
+ */
+ rt_mutex_dequeue(lock, waiter);
+ } else {
/*
- * We have to enqueue the top waiter(if it exists) into
- * task->pi_waiters list.
+ * If the lock has waiters already we check whether @task is
+ * eligible to take over the lock.
+ *
+ * If there are no other waiters, @task can acquire
+ * the lock. @task->pi_blocked_on is NULL, so it does
+ * not need to be dequeued.
*/
if (rt_mutex_has_waiters(lock)) {
- top = rt_mutex_top_waiter(lock);
- rt_mutex_enqueue_pi(task, top);
+ /*
+ * If @task->prio is greater than or equal to
+ * the top waiter priority (kernel view),
+ * @task lost.
+ */
+ if (task->prio >= rt_mutex_top_waiter(lock)->prio)
+ return 0;
+
+ /*
+ * The current top waiter stays enqueued. We
+ * don't have to change anything in the lock
+ * waiters order.
+ */
+ } else {
+ /*
+ * No waiters. Take the lock without the
+ * pi_lock dance.@task->pi_blocked_on is NULL
+ * and we have no waiters to enqueue in @task
+ * pi waiters list.
+ */
+ goto takeit;
}
- raw_spin_unlock_irqrestore(&task->pi_lock, flags);
}
+ /*
+ * Clear @task->pi_blocked_on. Requires protection by
+ * @task->pi_lock. Redundant operation for the @waiter == NULL
+ * case, but conditionals are more expensive than a redundant
+ * store.
+ */
+ raw_spin_lock_irqsave(&task->pi_lock, flags);
+ task->pi_blocked_on = NULL;
+ /*
+ * Finish the lock acquisition. @task is the new owner. If
+ * other waiters exist we have to insert the highest priority
+ * waiter into @task->pi_waiters list.
+ */
+ if (rt_mutex_has_waiters(lock))
+ rt_mutex_enqueue_pi(task, rt_mutex_top_waiter(lock));
+ raw_spin_unlock_irqrestore(&task->pi_lock, flags);
+
+takeit:
/* We got the lock. */
debug_rt_mutex_lock(lock);
+ /*
+ * This either preserves the RT_MUTEX_HAS_WAITERS bit if there
+ * are still waiters or clears it.
+ */
rt_mutex_set_owner(lock, task);
rt_mutex_deadlock_account_lock(lock, task);
{
struct task_struct *owner = rt_mutex_owner(lock);
struct rt_mutex_waiter *top_waiter = waiter;
- unsigned long flags;
+ struct rt_mutex *next_lock;
int chain_walk = 0, res;
+ unsigned long flags;
/*
* Early deadlock detection. We really don't want the task to
* which is wrong, as the other waiter is not in a deadlock
* situation.
*/
- if (detect_deadlock && owner == task)
+ if (owner == task)
return -EDEADLK;
raw_spin_lock_irqsave(&task->pi_lock, flags);
if (!owner)
return 0;
+ raw_spin_lock_irqsave(&owner->pi_lock, flags);
if (waiter == rt_mutex_top_waiter(lock)) {
- raw_spin_lock_irqsave(&owner->pi_lock, flags);
rt_mutex_dequeue_pi(owner, top_waiter);
rt_mutex_enqueue_pi(owner, waiter);
__rt_mutex_adjust_prio(owner);
if (owner->pi_blocked_on)
chain_walk = 1;
- raw_spin_unlock_irqrestore(&owner->pi_lock, flags);
- }
- else if (debug_rt_mutex_detect_deadlock(waiter, detect_deadlock))
+ } else if (debug_rt_mutex_detect_deadlock(waiter, detect_deadlock)) {
chain_walk = 1;
+ }
+
+ /* Store the lock on which owner is blocked or NULL */
+ next_lock = task_blocked_on_lock(owner);
- if (!chain_walk)
+ raw_spin_unlock_irqrestore(&owner->pi_lock, flags);
+ /*
+ * Even if full deadlock detection is on, if the owner is not
+ * blocked itself, we can avoid finding this out in the chain
+ * walk.
+ */
+ if (!chain_walk || !next_lock)
return 0;
/*
raw_spin_unlock(&lock->wait_lock);
- res = rt_mutex_adjust_prio_chain(owner, detect_deadlock, lock, waiter,
- task);
+ res = rt_mutex_adjust_prio_chain(owner, detect_deadlock, lock,
+ next_lock, waiter, task);
raw_spin_lock(&lock->wait_lock);
/*
* Wake up the next waiter on the lock.
*
- * Remove the top waiter from the current tasks waiter list and wake it up.
+ * Remove the top waiter from the current tasks pi waiter list and
+ * wake it up.
*
* Called with lock->wait_lock held.
*/
*/
rt_mutex_dequeue_pi(current, waiter);
- rt_mutex_set_owner(lock, NULL);
+ /*
+ * As we are waking up the top waiter, and the waiter stays
+ * queued on the lock until it gets the lock, this lock
+ * obviously has waiters. Just set the bit here and this has
+ * the added benefit of forcing all new tasks into the
+ * slow path making sure no task of lower priority than
+ * the top waiter can steal this lock.
+ */
+ lock->owner = (void *) RT_MUTEX_HAS_WAITERS;
raw_spin_unlock_irqrestore(¤t->pi_lock, flags);
+ /*
+ * It's safe to dereference waiter as it cannot go away as
+ * long as we hold lock->wait_lock. The waiter task needs to
+ * acquire it in order to dequeue the waiter.
+ */
wake_up_process(waiter->task);
}
{
int first = (waiter == rt_mutex_top_waiter(lock));
struct task_struct *owner = rt_mutex_owner(lock);
+ struct rt_mutex *next_lock = NULL;
unsigned long flags;
- int chain_walk = 0;
raw_spin_lock_irqsave(¤t->pi_lock, flags);
rt_mutex_dequeue(lock, waiter);
}
__rt_mutex_adjust_prio(owner);
- if (owner->pi_blocked_on)
- chain_walk = 1;
+ /* Store the lock on which owner is blocked or NULL */
+ next_lock = task_blocked_on_lock(owner);
raw_spin_unlock_irqrestore(&owner->pi_lock, flags);
}
- if (!chain_walk)
+ if (!next_lock)
return;
/* gets dropped in rt_mutex_adjust_prio_chain()! */
raw_spin_unlock(&lock->wait_lock);
- rt_mutex_adjust_prio_chain(owner, 0, lock, NULL, current);
+ rt_mutex_adjust_prio_chain(owner, 0, lock, next_lock, NULL, current);
raw_spin_lock(&lock->wait_lock);
}
void rt_mutex_adjust_pi(struct task_struct *task)
{
struct rt_mutex_waiter *waiter;
+ struct rt_mutex *next_lock;
unsigned long flags;
raw_spin_lock_irqsave(&task->pi_lock, flags);
raw_spin_unlock_irqrestore(&task->pi_lock, flags);
return;
}
-
+ next_lock = waiter->lock;
raw_spin_unlock_irqrestore(&task->pi_lock, flags);
/* gets dropped in rt_mutex_adjust_prio_chain()! */
get_task_struct(task);
- rt_mutex_adjust_prio_chain(task, 0, NULL, NULL, task);
+
+ rt_mutex_adjust_prio_chain(task, 0, NULL, next_lock, NULL, task);
}
/**
return ret;
}
+static void rt_mutex_handle_deadlock(int res, int detect_deadlock,
+ struct rt_mutex_waiter *w)
+{
+ /*
+ * If the result is not -EDEADLOCK or the caller requested
+ * deadlock detection, nothing to do here.
+ */
+ if (res != -EDEADLOCK || detect_deadlock)
+ return;
+
+ /*
+ * Yell lowdly and stop the task right here.
+ */
+ rt_mutex_print_deadlock(w);
+ while (1) {
+ set_current_state(TASK_INTERRUPTIBLE);
+ schedule();
+ }
+}
+
/*
* Slow path lock function:
*/
set_current_state(TASK_RUNNING);
- if (unlikely(ret))
+ if (unlikely(ret)) {
remove_waiter(lock, &waiter);
+ rt_mutex_handle_deadlock(ret, detect_deadlock, &waiter);
+ }
/*
* try_to_take_rt_mutex() sets the waiter bit
/*
* Slow path try-lock function:
*/
-static inline int
-rt_mutex_slowtrylock(struct rt_mutex *lock)
+static inline int rt_mutex_slowtrylock(struct rt_mutex *lock)
{
- int ret = 0;
+ int ret;
+
+ /*
+ * If the lock already has an owner we fail to get the lock.
+ * This can be done without taking the @lock->wait_lock as
+ * it is only being read, and this is a trylock anyway.
+ */
+ if (rt_mutex_owner(lock))
+ return 0;
+ /*
+ * The mutex has currently no owner. Lock the wait lock and
+ * try to acquire the lock.
+ */
raw_spin_lock(&lock->wait_lock);
- if (likely(rt_mutex_owner(lock) != current)) {
+ ret = try_to_take_rt_mutex(lock, current, NULL);
- ret = try_to_take_rt_mutex(lock, current, NULL);
- /*
- * try_to_take_rt_mutex() sets the lock waiters
- * bit unconditionally. Clean this up.
- */
- fixup_rt_mutex_waiters(lock);
- }
+ /*
+ * try_to_take_rt_mutex() sets the lock waiters bit
+ * unconditionally. Clean this up.
+ */
+ fixup_rt_mutex_waiters(lock);
raw_spin_unlock(&lock->wait_lock);
rt_mutex_deadlock_account_unlock(current);
- if (!rt_mutex_has_waiters(lock)) {
- lock->owner = NULL;
- raw_spin_unlock(&lock->wait_lock);
- return;
+ /*
+ * We must be careful here if the fast path is enabled. If we
+ * have no waiters queued we cannot set owner to NULL here
+ * because of:
+ *
+ * foo->lock->owner = NULL;
+ * rtmutex_lock(foo->lock); <- fast path
+ * free = atomic_dec_and_test(foo->refcnt);
+ * rtmutex_unlock(foo->lock); <- fast path
+ * if (free)
+ * kfree(foo);
+ * raw_spin_unlock(foo->lock->wait_lock);
+ *
+ * So for the fastpath enabled kernel:
+ *
+ * Nothing can set the waiters bit as long as we hold
+ * lock->wait_lock. So we do the following sequence:
+ *
+ * owner = rt_mutex_owner(lock);
+ * clear_rt_mutex_waiters(lock);
+ * raw_spin_unlock(&lock->wait_lock);
+ * if (cmpxchg(&lock->owner, owner, 0) == owner)
+ * return;
+ * goto retry;
+ *
+ * The fastpath disabled variant is simple as all access to
+ * lock->owner is serialized by lock->wait_lock:
+ *
+ * lock->owner = NULL;
+ * raw_spin_unlock(&lock->wait_lock);
+ */
+ while (!rt_mutex_has_waiters(lock)) {
+ /* Drops lock->wait_lock ! */
+ if (unlock_rt_mutex_safe(lock) == true)
+ return;
+ /* Relock the rtmutex and try again */
+ raw_spin_lock(&lock->wait_lock);
}
+ /*
+ * The wakeup next waiter path does not suffer from the above
+ * race. See the comments there.
+ */
wakeup_next_waiter(lock);
raw_spin_unlock(&lock->wait_lock);
return 1;
}
- ret = task_blocks_on_rt_mutex(lock, waiter, task, detect_deadlock);
+ /* We enforce deadlock detection for futexes */
+ ret = task_blocks_on_rt_mutex(lock, waiter, task, 1);
if (ret && !rt_mutex_owner(lock)) {
/*
projects / karo-tx-linux.git / blobdiff