From 6467822b8cc96e5feda98c7bf5c6329c6a896c91 Mon Sep 17 00:00:00 2001
From: Peter Zijlstra <peterz@infradead.org>
Date: Thu, 26 Aug 2021 09:36:53 +0200
Subject: [PATCH] locking/rtmutex: Prevent spurious EDEADLK return caused by
Git-commit: 6467822b8cc96e5feda98c7bf5c6329c6a896c91
Patch-mainline: v5.15-rc1
References: bsc#1190137 bsc#1189998
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
Signed-off-by: Davidlohr Bueso <dbueso@suse.de>
---
kernel/locking/rtmutex.c | 25 +++++++++++++++++++++++++
1 file changed, 25 insertions(+)
diff --git a/kernel/locking/rtmutex.c b/kernel/locking/rtmutex.c
index c8fe74ef8db9..3c1ba7b9a326 100644
--- 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;
+ /*
+ * 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
--
2.26.2