Messages in this thread | | | Date | Thu, 15 Oct 2015 17:52:56 +0200 | From | Frederic Weisbecker <> | Subject | Re: [PATCH 0/1] kmod: don't run async usermode helper as a child of kworker thread |
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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 > >
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