Kirill Tkhai <ktkhai@xxxxxxxxxxxxx> writes:
> On 02.05.2017 19:33, Oleg Nesterov wrote:
>> sorry for delay, vacation...
>>
>> On 04/28, Kirill Tkhai wrote:
>>>
>>> On 27.04.2017 19:22, Oleg Nesterov wrote:
>>>>
>>>> Ah, OK, I didn't notice the ns->child_reaper check in pidns_for_children_get().
>>>>
>>>> But note that it doesn't need tasklist_lock too.
>>>
>>> Hm, are there possible strange situations with memory ordering, when we see
>>> ns->child_reaper of already died ns, which was placed in the same memory?
>>> Do we have to use some memory barriers here?
>>
>> Could you spell please? I don't understand your concerns...
>>
>> I don't see how, say,
>>
>> static struct ns_common *pidns_for_children_get(struct task_struct *task)
>> {
>> struct ns_common *ns = NULL;
>> struct pid_namespace *pid_ns;
>>
>> task_lock(task);
>> if (task->nsproxy) {
>> pid_ns = task->nsproxy->pid_ns_for_children;
>> if (pid_ns->child_reaper) {
^^^^^^^^^^^^^^^^^^^^
Oleg my apologies I missed this line earlier.
This does look like a valid way to skip read_lock(&tasklist_lock);
>> ns = &pid_ns->ns;
>> get_pid_ns(ns);
^^^^^^^^^^^^^ This needs to be:
get_pid_ns(pid_ns);
>> }
>> }
>> task_unlock(task);
>>
>> return ns;
>> }
>>
>> can be wrong. It also looks more clean to me.
>>
>> ->child_reaper is not stable without tasklist, it can be dead/etc, but
>> we do not care?
>
> I mean the following. We had a pid_ns1 with a child_reaper set. Then
> it became dead, and a new pid_ns2 were allocated in the same memory.
task->nsproxy->pid_ns_for_children is always changed with
task_lock(task) held. See switch_task_namespaces (used by unshare and
setns). This also gives us the guarantee that the pid_ns reference
won't be freed/reused in any for until task_lock(task) is dropped.
> A task on another cpu opens the pid_for_children file, and because
> of there is no memory ordering, it sees pid_ns1->child_reaper,
> when it opens pid_ns2.
>
> I forgot, what guarantees this situation is impossible? What guarantees,
> the renewed content of pid_ns2 on another cpu is seen not later, than
> we can't open it?
Eric
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