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SubjectRe: [RFC][-mm] Add an owner to the mm_struct (v4)
KAMEZAWA Hiroyuki wrote:
> On Tue, 01 Apr 2008 18:13:12 +0530
> Balbir Singh <balbir@linux.vnet.ibm.com> wrote:
>> + /*
>> + * Search in the children
>> + */
>> + list_for_each_entry(c, &p->children, sibling) {
>> + if (c->mm == mm)
>> + goto assign_new_owner;
>> + }
>> +
> This finds new owner when "current" is multi-threaded and
> "current" called pthread_create(), right ?
>

No, it won't find the new owner if we have CLONE_THREAD passed while creating
threads. mm_need_new_owner() checks for !delay_group_leader(). If the
group_leader is set, we don't need a new owner, it stays around till all threads
exit.

>> + /*
>> + * Search in the siblings
>> + */
>> + list_for_each_entry(c, &p->parent->children, sibling) {
>> + if (c->mm == mm)
>> + goto assign_new_owner;
>> + }
>> +
> This finds new owner when "current" is multi-threaded and
> "current" is just a child (means it doesn't call pthread_create()) ?
>

Ditto

>
>> + /*
>> + * Search through everything else. We should not get
>> + * here often
>> + */
>> + do_each_thread(g, c) {
>> + if (c->mm == mm)
>> + goto assign_new_owner;
>> + } while_each_thread(g, c);
>
> Doing above in synchronized manner seems too heavy.
> When this happen ? or Can this be done in lazy "on-demand" manner ?
>

Do you mean under task_lock()?

> +assign_new_owner:
> + rcu_read_unlock();
> + BUG_ON(c == p);
> + task_lock(c);
> + if (c->mm != mm) {
> + task_unlock(c);
> + goto retry;
> + }
> + cgroup_mm_owner_callbacks(mm->owner, c);
> + mm->owner = c;
> + task_unlock(c);
> +}
> Why rcu_read_unlock() before changing owner ? Is it safe ?
>

It should be safe, since we take task_lock(), but to be doubly sure, we can drop
rcu read lock after taking the task_lock().

--
Warm Regards,
Balbir Singh
Linux Technology Center
IBM, ISTL


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