On Wed 01-06-16 00:29:33, Oleg Nesterov wrote:
> On 05/31, Michal Hocko wrote:
> >
> > On Mon 30-05-16 19:35:05, Oleg Nesterov wrote:
> > >
> > > Well, let me suggest this again. I think it should do
> > >
> > >
> > > if (SIGNAL_GROUP_COREDUMP)
> > > return false;
> > >
> > > if (SIGNAL_GROUP_EXIT)
> > > return true;
> > >
> > > if (thread_group_empty() && PF_EXITING)
> > > return true;
> > >
> > > return false;
> > >
> > > we do not need fatal_signal_pending(), in this case SIGNAL_GROUP_EXIT should
> > > be set (ignoring some bugs with sub-namespaces which we need to fix anyway).
> >
> > OK, so we shouldn't care about race when the fatal_signal is set on the
> > task until it reaches do_group_exit?
>
> if fatal_signal() is true then (ignoring exec and coredump) SIGNAL_GROUP_EXIT
> is already set (again, ignoring the bugs with sub-namespace inits).
>
> At the same time, SIGKILL can be already dequeued when the task exits, so
> fatal_signal_pending() can be "false negative".
Thanks for the clarification. I guess I got the point but this is a land
of surprises so one can never be sure...
> > > And. I think this needs smp_rmb() at the end of the loop (assuming we have the
> > > process_shares_mm() check here). We need it to ensure that we read p->mm before
> > > we read next_task(), to avoid the race with exit() + clone(CLONE_VM).
> >
> > Why don't we need the same barrier in oom_kill_process?
>
> Because it calls do_send_sig_info() which takes ->siglock and copy_process()
> takes the same lock. Not a barrier, but acts the same way.
Ahh ok, so an implicit barrier.
> > Which barrier it
> > would pair with?
>
> With the barrier implied by list_add_tail_rcu(&p->tasks, &init_task.tasks).
Ahh I see. rcu_assign_pointer that is, right?
> > Anyway I think this would deserve it's own patch.
> > Barriers are always tricky and it is better to have them in a small
> > patch with a full explanation.
>
> OK, agreed.
cool
> I am not sure I can read the new patch correctly, it depends on the previous
> changes... but afaics it looks good.
>
> Cosmetic/subjective nit, feel free to ignore,
>
> > +bool task_will_free_mem(struct task_struct *task)
> > +{
> > + struct mm_struct *mm = NULL;
>
> unnecessary initialization ;)
fixed
> > + struct task_struct *p;
> > + bool ret;
> > +
> > + /*
> > + * If the process has passed exit_mm we have to skip it because
> > + * we have lost a link to other tasks sharing this mm, we do not
> > + * have anything to reap and the task might then get stuck waiting
> > + * for parent as zombie and we do not want it to hold TIF_MEMDIE
> > + */
> > + p = find_lock_task_mm(task);
> > + if (!p)
> > + return false;
> > +
> > + if (!__task_will_free_mem(p)) {
> > + task_unlock(p);
> > + return false;
> > + }
>
> We can call the 1st __task_will_free_mem(p) before find_lock_task_mm(). In the
> likely case (I think) it should return false.
OK
>
> And since __task_will_free_mem() has no other callers perhaps it should go into
> oom_kill.c too.
ok
I will resend the whole series with the fixups later during this week.
Thanks again for your review.
--
Michal Hocko
SUSE Labs
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