В Вт, 16/06/2015 в 21:27 +0200, Oleg Nesterov пишет:
> On 06/16, Kirill Tkhai wrote:
> >
> > Second parameter of find_new_reaper() and the similarity of its name
> > and find_child_reaper()'s name confuse a reader.
>
> OK, I agree that
>
> reaper = find_child_reaper(father);
> ...
> reaper = find_new_reaper(father, reaper);
>
> can look confusing and probably deserves a cleanup. How about the patch
> below then?
Good, IMO it improves the readability.
>
> > Rename find_child_reaper() for better conformity of its name and its
> > function.
>
> I never argueus with renames ;) Probably the new name looks better.
>
> > Also delete the second parameter of find_new_reaper().
>
> Yes, we can do this. But this 2nd argument avoids another another
> task_active_pid_ns(father)->child_reaper, so this is optimization.
>
> I agree, this optimization is minor, but still I think this change
> needs some justification.
It looks like gcc inlines both of these function, so it seems there won't
be a problem...
>
> > +static struct task_struct *find_new_reaper(struct task_struct *father)
> > {
> > - struct task_struct *thread, *reaper;
> > + struct task_struct *thread, *reaper, *child_reaper;
> >
> > thread = find_alive_thread(father);
> > if (thread)
> > return thread;
> >
> > + child_reaper = task_active_pid_ns(father)->child_reaper;
> > + /*
> > + * child_reaper doesn't have children after zap_pid_ns_processes(),
> > + * therefore it can't enter this function.
> > + */
> > + BUG_ON(child_reaper == father);
>
> Yes, we can add this BUG_ON(). But please see the comments in
> zap_pid_ns_processes(). We can change zap_pid_ns_processes() so that
> it returns with non-empty ->children list due to EXIT_DEAD children.
Yes, I saw. Since zap_pid_ns_processes() waits for nr_hashed,
and __unhash_process() deletes from pid chain and sibling list
at the same time, pid_ns child_reaper can't have a child after
nr_hashed == init_pids.
> Unlikely we will actually do this, at least soon, so I won't argue
> with this BUG_ON().
>
> But. In this case it would be better to add it into forget_original_parent(),
>
> reaper = find_new_reaper(...);
> BUG_ON(reaper == father);
Yeah, I'm agree.
> Oh. Off-topic, but this reminds me that I forgot about another bug with
> ->has_child_subreaper... this needs another discussion.
>
> Oleg.
>
> --- x/kernel/exit.c
> +++ x/kernel/exit.c
> @@ -551,17 +551,17 @@ static void reparent_leader(struct task_
> static void forget_original_parent(struct task_struct *father,
> struct list_head *dead)
> {
> - struct task_struct *p, *t, *reaper;
> + struct task_struct *p, *t, *child_reaper, *reaper;
>
> if (unlikely(!list_empty(&father->ptraced)))
> exit_ptrace(father, dead);
>
> /* Can drop and reacquire tasklist_lock */
> - reaper = find_child_reaper(father);
> + child_reaper = find_child_reaper(father);
> if (list_empty(&father->children))
> return;
>
> - reaper = find_new_reaper(father, reaper);
> + reaper = find_new_reaper(father, child_reaper);
> list_for_each_entry(p, &father->children, sibling) {
> for_each_thread(p, t) {
> t->real_parent = reaper;
>
Thanks,
Kirill
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