rtmutex: Fix deadlock detector for real (397335f0) · Commits · 戴 / test

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>

kernel/locking/rtmutex.c

+28 −4

Original line number	Diff line number	Diff line
		@@ -343,9 +343,16 @@ static int rt_mutex_adjust_prio_chain(struct task_struct *task,
		* 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)))
		if (top_waiter) {
		if (!task_has_pi_waiters(task))
		goto out_unlock_pi;
		/*
		* If deadlock detection is off, we stop here if we
		* are not the top pi waiter of the task.
		*/
		if (!detect_deadlock && top_waiter != task_top_pi_waiter(task))
		goto out_unlock_pi;
		}

		/*
		* When deadlock detection is off then we check, if further
		@@ -361,7 +368,12 @@ static int rt_mutex_adjust_prio_chain(struct task_struct *task,
		goto retry;
		}

		/* Deadlock detection */
		/*
		* Deadlock detection. If the lock is the same as the original
		* lock which caused us to walk the lock chain or if the
		* current lock is owned by the task which initiated the chain
		* walk, we detected a deadlock.
		*/
		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);
		@@ -527,6 +539,18 @@ static int task_blocks_on_rt_mutex(struct rt_mutex *lock,
		unsigned long flags;
		int chain_walk = 0, res;

		/*
		* Early deadlock detection. We really don't want the task to
		* enqueue on itself just to untangle the mess later. It's not
		* only an optimization. We drop the locks, so another waiter
		* can come in before the chain walk detects the deadlock. So
		* the other will detect the deadlock and return -EDEADLOCK,
		* which is wrong, as the other waiter is not in a deadlock
		* situation.
		*/
		if (detect_deadlock && owner == task)
		return -EDEADLK;

		raw_spin_lock_irqsave(&task->pi_lock, flags);
		__rt_mutex_adjust_prio(task);
		waiter->task = task;

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