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(); } }