rtmutex: Fix deadlock detector for real
The current deadlock detection logic does not work reliably due to the following early exit path: /* * Drop out, when the task has no waiters. Note, * top_waiter can be NULL, when we are in the deboosting * mode! */ if (top_waiter && (!task_has_pi_waiters(task) || top_waiter != task_top_pi_waiter(task))) goto out_unlock_pi; So this not only exits when the task has no waiters, it also exits unconditionally when the current waiter is not the top priority waiter of the task. So in a nested locking scenario, it might abort the lock chain walk and therefor miss a potential deadlock. Simple fix: Continue the chain walk, when deadlock detection is enabled. We also avoid the whole enqueue, if we detect the deadlock right away (A-A). It's an optimization, but also prevents that another waiter who comes in after the detection and before the task has undone the damage observes the situation and detects the deadlock and returns -EDEADLOCK, which is wrong as the other task is not in a deadlock situation. Signed-off-by: Thomas Gleixner <tglx@linutronix.de> Cc: Peter Zijlstra <peterz@infradead.org> Reviewed-by: Steven Rostedt <rostedt@goodmis.org> Cc: Lai Jiangshan <laijs@cn.fujitsu.com> Cc: stable@vger.kernel.org Link: http://lkml.kernel.org/r/20140522031949.725272460@linutronix.de Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
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@ -343,9 +343,16 @@ static int rt_mutex_adjust_prio_chain(struct task_struct *task,
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* top_waiter can be NULL, when we are in the deboosting
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* mode!
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*/
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if (top_waiter && (!task_has_pi_waiters(task) ||
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top_waiter != task_top_pi_waiter(task)))
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if (top_waiter) {
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if (!task_has_pi_waiters(task))
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goto out_unlock_pi;
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/*
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* If deadlock detection is off, we stop here if we
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* are not the top pi waiter of the task.
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*/
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if (!detect_deadlock && top_waiter != task_top_pi_waiter(task))
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goto out_unlock_pi;
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}
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/*
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* When deadlock detection is off then we check, if further
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@ -361,7 +368,12 @@ static int rt_mutex_adjust_prio_chain(struct task_struct *task,
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goto retry;
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}
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/* Deadlock detection */
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/*
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* Deadlock detection. If the lock is the same as the original
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* lock which caused us to walk the lock chain or if the
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* current lock is owned by the task which initiated the chain
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* walk, we detected a deadlock.
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*/
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if (lock == orig_lock || rt_mutex_owner(lock) == top_task) {
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debug_rt_mutex_deadlock(deadlock_detect, orig_waiter, lock);
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raw_spin_unlock(&lock->wait_lock);
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@ -527,6 +539,18 @@ static int task_blocks_on_rt_mutex(struct rt_mutex *lock,
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unsigned long flags;
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int chain_walk = 0, res;
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/*
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* Early deadlock detection. We really don't want the task to
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* enqueue on itself just to untangle the mess later. It's not
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* only an optimization. We drop the locks, so another waiter
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* can come in before the chain walk detects the deadlock. So
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* the other will detect the deadlock and return -EDEADLOCK,
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* which is wrong, as the other waiter is not in a deadlock
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* situation.
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*/
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if (detect_deadlock && owner == task)
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return -EDEADLK;
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raw_spin_lock_irqsave(&task->pi_lock, flags);
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__rt_mutex_adjust_prio(task);
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waiter->task = task;
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