On 03/20, Michal Hocko wrote: > > [Cc Ingo and Chanho Min - the thread starts here > http://lkml.kernel.org/r/0000000000004cdec6058485b2ce@xxxxxxxxxx] > > On Wed 20-03-19 16:00:54, Oleg Nesterov wrote: > > On 03/20, Michal Hocko wrote: > > > > > > On Wed 20-03-19 14:24:11, Oleg Nesterov wrote: > > > > On 03/20, Michal Hocko wrote: > > > > > > > > > > Yes we do hold the cgred mutex while calling freezable_schedule but why > > > > > are we getting a warning is not really clear to me. The task should be > > > > > hidden from the freezer so why do we warn at all? > > > > > > > > try_to_freeze() calls debug_check_no_locks_held() and this makes sense. > > > > > > Yes it does. But it already ignores PF_NOFREEZE tasks and I fail to see > > > why is PF_FREEZER_SKIP any different. > > > > But they differ. PF_NOFREEZE is a "sticky" flag for kthreads. Set by default, > > cleared by set_freezable() if you want a freezable kthread. > > > > PF_FREEZER_SKIP means that a sleeping freezable task will call try_to_freeze() > > right after schedule() returns, so try_to_freeze_tasks() can safely count it as > > "already frozen". > > But the fundamental semantic is the same right? Both might be sitting on > locks that might interfere with other tasks and we should be _extra_ > careful when using them. In an ideal world, none of them is really > needed. Ah, it seems that we misunderstood each other... see below. > So my question remains. Can we drop the warning for PF_FREEZER_SKIP > tasks as well? But why? It is obviously wrong to call try_to_freeze() with a lock held. Probably you meant the if (!(current->flags & PF_NOFREEZE)) check in try_to_freeze() when you said "already ignores PF_NOFREEZE tasks". I am not sure we actually need this check, a PF_NOFREEZE kthread shouldn't call try_to_freeze() at least directly. However, note that freezing() will return false if PF_NOFREEZE is set, so try_to_freeze() is nop in this case. Probably this is why PF_NOFREEZE is also checked before debug_check_no_locks_held(). > > > as removing the cgred is way way too complicated. > > > > We need to do this anyway, this leads to other more serious problems... > > Yes but this is far away and it doesn't really seem like a stable tree > material strace -f can hang ;) so this is the stable material. Oleg.
