On 10/15, Frederic Weisbecker wrote:
>
> On Thu, Oct 15, 2015 at 04:37:57PM +0200, Oleg Nesterov wrote:
> > call_usermodehelper_exec_sync() does fork() + wait() with "unignored"
> > SIGCHLD. What we have missed is that this worker thread can have other
> > children previously forked by call_usermodehelper_exec_work() without
> > UMH_WAIT_PROC. If such a child exits in between it becomes a zombie and
> > nobody can reap it (unless/until this worker thread exits too).
>
> I think we should elaborate a tiny bit the last sentence here:
OK, I'll try to update the changelog and send v2...
> "When the parent masks SIGCHLD, a child autoreaps itself, this is
> what we expect from !UMH_WAIT_PROC children.
^^^^^^^^^^^^^^^^^^^^^^^
Not really. This is what we _usually_ expect from kernel_thread().
> > @@ -327,9 +327,13 @@ static void call_usermodehelper_exec_work(struct
> > work_struct *work)
> > call_usermodehelper_exec_sync(sub_info);
> > } else {
> > pid_t pid;
> > -
> > + /*
> > + * Use CLONE_PARENT to reparent it to kthreadd; we do not
> > + * want to pollute current->children, in particular because
> > + * call_usermodehelper_exec_sync() assumes it is empty.
> > + */
>
> IMHO, that too should get some more details. Maybe:
>
> + /*
> + * Use CLONE_PARENT to reparent it to kthreadd. We need a parent
> + * that always ignore SIGCHLD such that the child always
> autoreaps
> + * as expected.
> + */
Well, OK...
But I would like to keep "we do not want to pollute current->children"
because this the goal of the next cleanups.
Plus I don't really like "parent that always ignore SIGCHLD". To remind,
we can also remove kernel_sigaction() and sys_wait4() from
call_usermodehelper_exec_sync(). Plus I have other changes in mind,
kernel_thread() should not rely on SIGCHLD at all. The auto-reapable
kernel threads should run with ->exit_signal == 0.
Finally, this comment should go into kernel_thread() eventually.
Oleg.
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