On Mon, 2019-01-07 at 17:50 +0100, Sebastian Andrzej Siewior wrote: > On 2019-01-04 15:33:21 [-0500], Scott Wood wrote: > > __down_read() bypasses the rtmutex frontend to call > > rt_mutex_slowlock_locked() directly, and thus it needs to call > > blk_schedule_flush_flug() itself. > > we don't do this in the spin_lock() case because !RT doesn't do it. And because spin_lock() is called inside the flush path. > We > do it for rtmutex because !RT does it for mutex. > Now I can't remember why this was skipped for a rw_sem since it is > performed for !RT as part of the schedule() invocation. Without this we were seeing XFS hangs on our internal kernel. I wasn't able to reproduce it on a newer kernel, but it's very timing-dependant so I wouldn't read too much into that. > If I don't come up with a plausible explanation then I will apply this > plus a hunk for the __down_write_common() case which should also be > required (right?). I don't think it's needed, as it doesn't call into the rtmutex code via a backdoor. When blocking on sem->rtmutex, rt_mutex_fastlock() will call the flush. When blocking with a direct call to schedule(), tsk_is_pi_blocked() will not be true, and thus schedule() will do the flush via sched_submit_work(). -Scott
