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Commit 397335f0 authored by Thomas Gleixner's avatar Thomas Gleixner
Browse files

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: default avatarThomas Gleixner <tglx@linutronix.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Reviewed-by: default avatarSteven 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: default avatarThomas Gleixner <tglx@linutronix.de>
parent f0d71b3d

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;