On Wed 20-03-19 18:30:42, Oleg Nesterov wrote: > 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? To drop the warning which led to the revert. > It is obviously wrong to call try_to_freeze() with a lock held. It is but the question what do we care about more. A task blocking suspend so that the operation fails or a process being frozen with cgred held. We have discussed that during the review of the original patch and concluded that this is the only way for now AFAIR. > Probably you meant the > > if (!(current->flags & PF_NOFREEZE)) > > check in try_to_freeze() when you said "already ignores PF_NOFREEZE tasks". Yes, that is the check I've had in mind. -- Michal Hocko SUSE Labs
