On Thu, Dec 20, 2018 at 05:16:50PM +0100, Oleg Nesterov wrote: > On 12/18, Roman Gushchin wrote: > > > > > > > > --- a/kernel/freezer.c > > > > > > +++ b/kernel/freezer.c > > > > > > @@ -134,7 +134,7 @@ bool freeze_task(struct task_struct *p) > > > > > > return false; > > > > > > > > > > > > spin_lock_irqsave(&freezer_lock, flags); > > > > > > - if (!freezing(p) || frozen(p)) { > > > > > > + if (!freezing(p) || frozen(p) || cgroup_task_frozen()) { > > > > > > spin_unlock_irqrestore(&freezer_lock, flags); > > > > > > return false; > > > > > > } > > > > > > > > > > > > -- > > > > > > > > > > > > If the task is already frozen by the cgroup freezer, we don't have to do > > > > > > anything additionally. > > > > > > > > > > I don't think so. A cgroup_task_frozen() task can be killed after > > > > > try_to_freeze_tasks() succeeds, and the exiting task can close files, > > > > > do IO, etc. Or it can be thawed by cgroup_freeze_task(false). > > > > > > > > > > In short, if try_to_freeze_tasks() succeeds, the caller has all rights > > > > > to assume that nobody can escape from __refrigerator(). > > > > > > > > But this is what we do with stopped and ptraced tasks, isn't it? > > > > > > No, > > > > > > > We do use freezable_schedule() and the system freezer just ignores such tasks. > > > > > > static inline void freezable_schedule(void) > > > { > > > freezer_do_not_count(); > > > schedule(); > > > freezer_count(); > > > } > > > > > > and note that freezer_count() calls try_to_freeze(). > > > > > > IOW, the task sleeping in freezable_schedule() doesn't really differ from the > > > task sleeping in __refrigerator(). It is not that "the system freezer just > > > ignores such tasks", it ignores them because it can safely count them as frozen. > > > > Right, so the task is sleeping peacefully, and we know, that it won't get > > anywhere, because we'll catch it in freezer_count(). We allow it to sleep > > there, we don't force it to __refrigerator(), and we treat it as frozen. > > > > How's that different to cgroup v2 freezer? If the task is frozen by cgroup v2 > > freezer, let it sleep there, and catch if it tries to escape. Exactly as it > > works for SIGSTOP. > > > > Am I missing something? > > Roman, perhaps we misunderstood each other... > > I still think that the cgroup_task_frozen() check in freeze_task() you proposed > a) is not right, and b) it is not what we do with the STOPPED/TRACED tasks which > call freezable_schedule(). This is what I tried to say. > > If you meant that freezer v2 can too use freezable_schedule() - I agree. Sorry for the confusion. Yeah, what I'm saying, is that freezable_schedule() will work for v2 as well. > > > So, you think that v2 freezer should follow the same approach, and allow tasks > > sleeping on SIGSTOP, for instance, to be treated as frozen? > > Hm, maybe. I have to think more here. > > I think this would be nice. Otherwise, say, CGRP_FREEZE can be never reported > if I read this code correctly. And this looks "symmetrical" with the fact that > a ->frozen task reacts to SIGSTOP and it is still treated as frozen after that. Yeah, looks so. I'll try to implement this in v6. Thanks!