On 02.05.2017 19:33, Oleg Nesterov wrote:
sorry for delay, vacation...
On 04/28, Kirill Tkhai wrote:
quoted
On 27.04.2017 19:22, Oleg Nesterov wrote:
quoted
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) {
ns = &pid_ns->ns;
get_pid_ns(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.
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?