From a055fcc132d4c25b96d1115aea514258810dc6fc Mon Sep 17 00:00:00 2001 From: Peter Zijlstra Date: Thu, 26 Aug 2021 10:48:18 +0200 Subject: [PATCH] locking/rtmutex: Return success on deadlock for ww_mutex 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 Fixes: add461325ec5 ("locking/rtmutex: Extend the rtmutex core to support ww_mutex") Signed-off-by: Peter Zijlstra (Intel) Signed-off-by: Thomas Gleixner Link: https://lore.kernel.org/r/YSeWjCHoK4v5OcOt@hirez.programming.kicks-ass.net --- 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; } -- 2.20.1