On Thu, Dec 01, 2016 at 03:06:45PM +0100, Nicolai Hähnle wrote: > From: Nicolai Hähnle <nicolai.haeh...@amd.com> > > In the following scenario, thread #1 should back off its attempt to lock > ww1 and unlock ww2 (assuming the acquire context stamps are ordered > accordingly). > > Thread #0 Thread #1 > --------- --------- > successfully lock ww2 > set ww1->base.owner > attempt to lock ww1 > confirm ww1->ctx == NULL > enter mutex_spin_on_owner > set ww1->ctx > > What was likely to happen previously is: > > attempt to lock ww2 > refuse to spin because > ww2->ctx != NULL > schedule() > detect thread #0 is off CPU > stop optimistic spin > return -EDEADLK > unlock ww2 > wakeup thread #0 > lock ww2 > > Now, we are more likely to see: > > detect ww1->ctx != NULL > stop optimistic spin > return -EDEADLK > unlock ww2 > successfully lock ww2 > > ... because thread #1 will stop its optimistic spin as soon as possible. > > The whole scenario is quite unlikely, since it requires thread #1 to get > between thread #0 setting the owner and setting the ctx. But since we're > idling here anyway, the additional check is basically free. > > Found by inspection.
Similar question can be raised for can_spin_on_owner() as well. Is it worth for a contending ww_mutex to enter the osq queue if we expect a EDEADLK? It seems to boil down to how likely is the EDEADLK going to evaporate if we wait for the owner to finish and unlock. The patch looks reasonable, just a question of desirability. -Chris -- Chris Wilson, Intel Open Source Technology Centre