rtmutex: Handle the various new futex race conditions
RT opens a few new interesting race conditions in the rtmutex/futex combo due to futex hash bucket lock being a 'sleeping' spinlock and therefor not disabling preemption. Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
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@ -2270,6 +2270,16 @@ retry_private:
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requeue_pi_wake_futex(this, &key2, hb2);
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drop_count++;
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continue;
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} else if (ret == -EAGAIN) {
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/*
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* Waiter was woken by timeout or
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* signal and has set pi_blocked_on to
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* PI_WAKEUP_INPROGRESS before we
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* tried to enqueue it on the rtmutex.
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*/
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this->pi_state = NULL;
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put_pi_state(pi_state);
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continue;
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} else if (ret) {
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/*
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* rt_mutex_start_proxy_lock() detected a
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@ -3316,7 +3326,7 @@ static int futex_wait_requeue_pi(u32 __user *uaddr, unsigned int flags,
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{
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struct hrtimer_sleeper timeout, *to;
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struct rt_mutex_waiter rt_waiter;
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struct futex_hash_bucket *hb;
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struct futex_hash_bucket *hb, *hb2;
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union futex_key key2 = FUTEX_KEY_INIT;
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struct futex_q q = futex_q_init;
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int res, ret;
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@ -3368,20 +3378,55 @@ static int futex_wait_requeue_pi(u32 __user *uaddr, unsigned int flags,
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/* Queue the futex_q, drop the hb lock, wait for wakeup. */
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futex_wait_queue_me(hb, &q, to);
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/*
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* On RT we must avoid races with requeue and trying to block
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* on two mutexes (hb->lock and uaddr2's rtmutex) by
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* serializing access to pi_blocked_on with pi_lock.
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*/
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raw_spin_lock_irq(¤t->pi_lock);
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if (current->pi_blocked_on) {
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/*
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* We have been requeued or are in the process of
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* being requeued.
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*/
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raw_spin_unlock_irq(¤t->pi_lock);
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} else {
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/*
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* Setting pi_blocked_on to PI_WAKEUP_INPROGRESS
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* prevents a concurrent requeue from moving us to the
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* uaddr2 rtmutex. After that we can safely acquire
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* (and possibly block on) hb->lock.
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*/
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current->pi_blocked_on = PI_WAKEUP_INPROGRESS;
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raw_spin_unlock_irq(¤t->pi_lock);
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spin_lock(&hb->lock);
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/*
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* Clean up pi_blocked_on. We might leak it otherwise
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* when we succeeded with the hb->lock in the fast
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* path.
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*/
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raw_spin_lock_irq(¤t->pi_lock);
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current->pi_blocked_on = NULL;
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raw_spin_unlock_irq(¤t->pi_lock);
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ret = handle_early_requeue_pi_wakeup(hb, &q, &key2, to);
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spin_unlock(&hb->lock);
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if (ret)
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goto out_put_keys;
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}
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/*
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* In order for us to be here, we know our q.key == key2, and since
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* we took the hb->lock above, we also know that futex_requeue() has
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* completed and we no longer have to concern ourselves with a wakeup
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* race with the atomic proxy lock acquisition by the requeue code. The
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* futex_requeue dropped our key1 reference and incremented our key2
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* reference count.
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* In order to be here, we have either been requeued, are in
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* the process of being requeued, or requeue successfully
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* acquired uaddr2 on our behalf. If pi_blocked_on was
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* non-null above, we may be racing with a requeue. Do not
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* rely on q->lock_ptr to be hb2->lock until after blocking on
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* hb->lock or hb2->lock. The futex_requeue dropped our key1
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* reference and incremented our key2 reference count.
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*/
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hb2 = hash_futex(&key2);
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/* Check if the requeue code acquired the second futex for us. */
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if (!q.rt_waiter) {
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@ -3390,14 +3435,15 @@ static int futex_wait_requeue_pi(u32 __user *uaddr, unsigned int flags,
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* did a lock-steal - fix up the PI-state in that case.
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*/
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if (q.pi_state && (q.pi_state->owner != current)) {
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spin_lock(q.lock_ptr);
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spin_lock(&hb2->lock);
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BUG_ON(&hb2->lock != q.lock_ptr);
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ret = fixup_pi_state_owner(uaddr2, &q, current);
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/*
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* Drop the reference to the pi state which
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* the requeue_pi() code acquired for us.
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*/
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put_pi_state(q.pi_state);
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spin_unlock(q.lock_ptr);
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spin_unlock(&hb2->lock);
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/*
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* Adjust the return value. It's either -EFAULT or
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* success (1) but the caller expects 0 for success.
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@ -3416,7 +3462,8 @@ static int futex_wait_requeue_pi(u32 __user *uaddr, unsigned int flags,
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pi_mutex = &q.pi_state->pi_mutex;
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ret = rt_mutex_wait_proxy_lock(pi_mutex, to, &rt_waiter);
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spin_lock(q.lock_ptr);
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spin_lock(&hb2->lock);
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BUG_ON(&hb2->lock != q.lock_ptr);
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if (ret && !rt_mutex_cleanup_proxy_lock(pi_mutex, &rt_waiter))
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ret = 0;
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@ -136,6 +136,11 @@ static void fixup_rt_mutex_waiters(struct rt_mutex *lock)
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WRITE_ONCE(*p, owner & ~RT_MUTEX_HAS_WAITERS);
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}
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static int rt_mutex_real_waiter(struct rt_mutex_waiter *waiter)
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{
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return waiter && waiter != PI_WAKEUP_INPROGRESS;
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}
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/*
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* We can speed up the acquire/release, if there's no debugging state to be
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* set up.
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@ -380,7 +385,8 @@ int max_lock_depth = 1024;
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static inline struct rt_mutex *task_blocked_on_lock(struct task_struct *p)
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{
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return p->pi_blocked_on ? p->pi_blocked_on->lock : NULL;
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return rt_mutex_real_waiter(p->pi_blocked_on) ?
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p->pi_blocked_on->lock : NULL;
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}
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/*
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@ -516,7 +522,7 @@ static int rt_mutex_adjust_prio_chain(struct task_struct *task,
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* reached or the state of the chain has changed while we
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* dropped the locks.
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*/
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if (!waiter)
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if (!rt_mutex_real_waiter(waiter))
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goto out_unlock_pi;
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/*
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@ -950,6 +956,22 @@ static int task_blocks_on_rt_mutex(struct rt_mutex *lock,
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return -EDEADLK;
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raw_spin_lock(&task->pi_lock);
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/*
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* In the case of futex requeue PI, this will be a proxy
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* lock. The task will wake unaware that it is enqueueed on
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* this lock. Avoid blocking on two locks and corrupting
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* pi_blocked_on via the PI_WAKEUP_INPROGRESS
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* flag. futex_wait_requeue_pi() sets this when it wakes up
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* before requeue (due to a signal or timeout). Do not enqueue
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* the task if PI_WAKEUP_INPROGRESS is set.
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*/
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if (task != current && task->pi_blocked_on == PI_WAKEUP_INPROGRESS) {
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raw_spin_unlock(&task->pi_lock);
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return -EAGAIN;
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}
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BUG_ON(rt_mutex_real_waiter(task->pi_blocked_on));
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waiter->task = task;
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waiter->lock = lock;
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waiter->prio = task->prio;
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@ -973,7 +995,7 @@ static int task_blocks_on_rt_mutex(struct rt_mutex *lock,
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rt_mutex_enqueue_pi(owner, waiter);
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rt_mutex_adjust_prio(owner);
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if (owner->pi_blocked_on)
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if (rt_mutex_real_waiter(owner->pi_blocked_on))
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chain_walk = 1;
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} else if (rt_mutex_cond_detect_deadlock(waiter, chwalk)) {
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chain_walk = 1;
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@ -1069,7 +1091,7 @@ static void remove_waiter(struct rt_mutex *lock,
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{
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bool is_top_waiter = (waiter == rt_mutex_top_waiter(lock));
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struct task_struct *owner = rt_mutex_owner(lock);
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struct rt_mutex *next_lock;
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struct rt_mutex *next_lock = NULL;
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lockdep_assert_held(&lock->wait_lock);
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@ -1095,6 +1117,7 @@ static void remove_waiter(struct rt_mutex *lock,
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rt_mutex_adjust_prio(owner);
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/* Store the lock on which owner is blocked or NULL */
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if (rt_mutex_real_waiter(owner->pi_blocked_on))
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next_lock = task_blocked_on_lock(owner);
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raw_spin_unlock(&owner->pi_lock);
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@ -1131,7 +1154,8 @@ void rt_mutex_adjust_pi(struct task_struct *task)
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raw_spin_lock_irqsave(&task->pi_lock, flags);
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waiter = task->pi_blocked_on;
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if (!waiter || rt_mutex_waiter_equal(waiter, task_to_waiter(task))) {
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if (!rt_mutex_real_waiter(waiter) ||
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rt_mutex_waiter_equal(waiter, task_to_waiter(task))) {
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raw_spin_unlock_irqrestore(&task->pi_lock, flags);
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return;
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}
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@ -130,6 +130,8 @@ enum rtmutex_chainwalk {
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/*
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* PI-futex support (proxy locking functions, etc.):
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*/
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#define PI_WAKEUP_INPROGRESS ((struct rt_mutex_waiter *) 1)
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extern struct task_struct *rt_mutex_next_owner(struct rt_mutex *lock);
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extern void rt_mutex_init_proxy_locked(struct rt_mutex *lock,
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struct task_struct *proxy_owner);
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