Commit 6467822b authored by Peter Zijlstra's avatar Peter Zijlstra Committed by Thomas Gleixner
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locking/rtmutex: Prevent spurious EDEADLK return caused by ww_mutexes 



rtmutex based ww_mutexes can legitimately create a cycle in the lock graph
which can be observed by a blocker which didn't cause the problem:

   P1: A, ww_A, ww_B
   P2: ww_B, ww_A
   P3: A

P3 might therefore be trapped in the ww_mutex induced cycle and run into
the lock depth limitation of rt_mutex_adjust_prio_chain() which returns
-EDEADLK to the caller.

Disable the deadlock detection walk when the chain walk observes a
ww_mutex to prevent this looping.

[ tglx: Split it apart and added changelog ]

Reported-by: default avatarSebastian Siewior <bigeasy@linutronix.de>
Fixes: add46132 ("locking/rtmutex: Extend the rtmutex core to support ww_mutex")
Signed-off-by: default avatarPeter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: default avatarThomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/r/YSeWjCHoK4v5OcOt@hirez.programming.kicks-ass.net
parent 37e8abff

kernel/locking/rtmutex.c

+25 −0
Original line number Diff line number Diff line
@@ -656,6 +656,31 @@ static int __sched rt_mutex_adjust_prio_chain(struct task_struct *task, 
	if (next_lock != waiter->lock)

		goto out_unlock_pi;



	/*

	 * There could be 'spurious' loops in the lock graph due to ww_mutex,

	 * consider:

	 *

	 *   P1: A, ww_A, ww_B

	 *   P2: ww_B, ww_A

	 *   P3: A

	 *

	 * P3 should not return -EDEADLK because it gets trapped in the cycle

	 * created by P1 and P2 (which will resolve -- and runs into

	 * max_lock_depth above). Therefore disable detect_deadlock such that

	 * the below termination condition can trigger once all relevant tasks

	 * are boosted.

	 *

	 * Even when we start with ww_mutex we can disable deadlock detection,

	 * since we would supress a ww_mutex induced deadlock at [6] anyway.

	 * Supressing it here however is not sufficient since we might still

	 * hit [6] due to adjustment driven iteration.

	 *

	 * NOTE: if someone were to create a deadlock between 2 ww_classes we'd

	 * utterly fail to report it; lockdep should.

	 */

	if (IS_ENABLED(CONFIG_PREEMPT_RT) && waiter->ww_ctx && detect_deadlock)

		detect_deadlock = false;



	/*

	 * Drop out, when the task has no waiters. Note,

	 * top_waiter can be NULL, when we are in the deboosting