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: Sebastian Siewior <bigeasy@linutronix.de>
Fixes: add461325ec5 ("locking/rtmutex: Extend the rtmutex core to support ww_mutex")
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/r/YSeWjCHoK4v5OcOt@hirez.programming.kicks-ass.net

diff --git a/kernel/locking/rtmutex.c b/kernel/locking/rtmutex.c
index c8fe74e..3c1ba7b 100644 (file)
--- a/kernel/locking/rtmutex.c
+++ b/kernel/locking/rtmutex.c
@@ -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;
 
        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
        /*
         * Drop out, when the task has no waiters. Note,
         * top_waiter can be NULL, when we are in the deboosting