[tip: locking/core] locking/rtmutex: Prevent spurious EDEADLK return caused by ww_mutexes

[tip-bot2 for Peter Zijlstra]

The following commit has been merged into the locking/core branch of tip:

Commit-ID:     6467822b8cc96e5feda98c7bf5c6329c6a896c91
Gitweb:        https://git.kernel.org/tip/6467822b8cc96e5feda98c7bf5c6329c6a896c91
Author:        Peter Zijlstra <peterz@xxxxxxxxxxxxx>
AuthorDate:    Thu, 26 Aug 2021 09:36:53 +02:00
Committer:     Thomas Gleixner <tglx@xxxxxxxxxxxxx>
CommitterDate: Fri, 27 Aug 2021 14:28:49 +02:00

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@xxxxxxxxxxxxx>
Fixes: add461325ec5 ("locking/rtmutex: Extend the rtmutex core to support ww_mutex")
Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Link: https://lore.kernel.org/r/YSeWjCHoK4v5OcOt@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
---
 kernel/locking/rtmutex.c | 25 +++++++++++++++++++++++++
 1 file changed, 25 insertions(+)

diff --git a/kernel/locking/rtmutex.c b/kernel/locking/rtmutex.c
index c8fe74e..3c1ba7b 100644
--- a/kernel/locking/rtmutex.c
+++ b/kernel/locking/rtmutex.c
@@ -657,6 +657,31 @@ static int __sched rt_mutex_adjust_prio_chain(struct task_struct *task,
 		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
 	 * mode!