On Sat, Apr 09, 2016 at 11:25:39AM +0800, Xunlei Pang wrote: > > In any case, I just realized we do not in fact provide this guarantee > > (of pointing to a blocked task) that needs a bit more work. > > Current patch calls rt_mutex_adjust_prio() before wake_up_q() the > wakee, at that moment the wakee has been removed by > rt_mutex_slowunlock()->mark_wakeup_next_waiter(), from current's > pi_waiters, rt_mutex_adjust_prio() won't see this wakee, so I think > this should not be problem.
No, any wakeup after mark_wakeup_next_waiter() will make the task run. And since we must always consider spurious wakeups, we cannot rely on us (eg. our wake_up_q call) being the one and only. Therefore it is possible and the only thing that stands between us and doom is the fact that the wake_q stuff holds a task reference. But we cannot guarantee that the task we have a pointer to is in fact blocked.
