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SubjectRe: [PATCH] sched/deadline/rtmutex: Fix a PI crash for deadline tasks
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();
}
}

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