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:
"When the parent masks SIGCHLD, a child autoreaps itself, this is
what we expect from !UMH_WAIT_PROC children. Now if such a child exits during
this unlucky window where the parent worker enabled SIGCHLD to wait for a
sibling,
the autoreap will fail and the child then becomes a zombie because nobody can
reap it
(unless/until this worker thread exits too)."
>
> Change the !UMH_WAIT_PROC case to use CLONE_PARENT.
>
> Note: this is only first step. All PF_KTHREAD tasks, even created by
> kernel_thread() should have ->parent == kthreadd by default.
>
> Signed-off-by: Oleg Nesterov <o...@redhat.com>
> ---
> kernel/kmod.c | 8 ++++++--
> 1 file changed, 6 insertions(+), 2 deletions(-)
>
> diff --git a/kernel/kmod.c b/kernel/kmod.c
> index da98d05..e7185a2 100644
> --- a/kernel/kmod.c
> +++ b/kernel/kmod.c
> @@ -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.
+ */
Thanks!
> pid = kernel_thread(call_usermodehelper_exec_async, sub_info,
> - SIGCHLD);
> + CLONE_PARENT | SIGCHLD);
> if (pid < 0) {
> sub_info->retval = pid;
> umh_complete(sub_info);
> --
> 2.4.3
>
>
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