Re: [PATCH 0/1] kmod: don't run async usermode helper as a child of kworker thread

[Frederic Weisbecker <fweisbec@gmail.com>]

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 <oleg@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
> 
>