Re: [PATCH] sched/deadline/rtmutex: Fix a PI crash for deadline tasks

[Peter Zijlstra <peterz@infradead.org>]

On Tue, Apr 05, 2016 at 11:19:54AM +0200, Peter Zijlstra wrote:
> Or did I miss something (again) ? :-)
> 
> ---
>  kernel/locking/rtmutex.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/kernel/locking/rtmutex.c b/kernel/locking/rtmutex.c
> index 3e746607abe5..36eb232bd29f 100644
> --- a/kernel/locking/rtmutex.c
> +++ b/kernel/locking/rtmutex.c
> @@ -1390,11 +1390,11 @@ rt_mutex_fastunlock(struct rt_mutex *lock,
>  	} else {
>  		bool deboost = slowfn(lock, &wake_q);
>  
> -		wake_up_q(&wake_q);
> -
>  		/* Undo pi boosting if necessary: */
>  		if (deboost)
>  			rt_mutex_adjust_prio(current);
> +
> +		wake_up_q(&wake_q);
>  	}
>  }

So one potential issue with this -- and this might be reason this code
is the way it is -- is that the moment we de-boost we can get preempted,
before having had a chance to wake the higher prio task, getting
ourselves into a prio-inversion.

But again, that should be fairly simply to fix.

--
 kernel/locking/rtmutex.c | 16 +++++++++++++---
 1 file changed, 13 insertions(+), 3 deletions(-)

diff --git a/kernel/locking/rtmutex.c b/kernel/locking/rtmutex.c
index 3e746607abe5..1896baf28e9c 100644
--- a/kernel/locking/rtmutex.c
+++ b/kernel/locking/rtmutex.c
@@ -1390,11 +1390,21 @@ rt_mutex_fastunlock(struct rt_mutex *lock,
 	} else {
 		bool deboost = slowfn(lock, &wake_q);
 
-		wake_up_q(&wake_q);
-
-		/* Undo pi boosting if necessary: */
+		/*
+		 * Undo pi boosting (if necessary) and wake top waiter.
+		 *
+		 * We should deboost before waking the high-prio task such that
+		 * we don't run two tasks with the 'same' state. This however
+		 * can lead to prio-inversion if we would get preempted after
+		 * the deboost but before waking our high-prio task, hence the
+		 * preempt_disable.
+		 */
+		preempt_disable();
 		if (deboost)
 			rt_mutex_adjust_prio(current);
+
+		wake_up_q(&wake_q);
+		preempt_enable();
 	}
 }