From a055fcc132d4c25b96d1115aea514258810dc6fc Mon Sep 17 00:00:00 2001
From: Peter Zijlstra <peterz@infradead.org>
Date: Thu, 26 Aug 2021 10:48:18 +0200
Subject: [PATCH] locking/rtmutex: Return success on deadlock for ww_mutex
Git-commit: a055fcc132d4c25b96d1115aea514258810dc6fc
Patch-mainline: v5.15-rc1
References: bsc#1190137 bsc#1189998
waiters
ww_mutexes can legitimately cause a deadlock situation in the lock graph
which is resolved afterwards by the wait/wound mechanics. The rtmutex chain
walk can detect such a deadlock and returns EDEADLK which in turn skips the
wait/wound mechanism and returns EDEADLK to the caller. That's wrong
because both lock chains might get EDEADLK or the wrong waiter would back
out.
Detect that situation and return 'success' in case that the waiter which
initiated the chain walk is a ww_mutex with context. This allows the
wait/wound mechanics to resolve the situation according to the rules.
[ 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 | 15 ++++++++++++++-
1 file changed, 14 insertions(+), 1 deletion(-)
diff --git a/kernel/locking/rtmutex.c b/kernel/locking/rtmutex.c
index 3c1ba7b9a326..8eabdc79602b 100644
--- a/kernel/locking/rtmutex.c
+++ b/kernel/locking/rtmutex.c
@@ -742,8 +742,21 @@ static int __sched rt_mutex_adjust_prio_chain(struct task_struct *task,
* walk, we detected a deadlock.
*/
if (lock == orig_lock || rt_mutex_owner(lock) == top_task) {
- raw_spin_unlock(&lock->wait_lock);
ret = -EDEADLK;
+
+ /*
+ * When the deadlock is due to ww_mutex; also see above. Don't
+ * report the deadlock and instead let the ww_mutex wound/die
+ * logic pick which of the contending threads gets -EDEADLK.
+ *
+ * NOTE: assumes the cycle only contains a single ww_class; any
+ * other configuration and we fail to report; also, see
+ * lockdep.
+ */
+ if (IS_ENABLED(CONFIG_PREEMPT_RT) && orig_waiter->ww_ctx)
+ ret = 0;
+
+ raw_spin_unlock(&lock->wait_lock);
goto out_unlock_pi;
}
--
2.26.2