linux-kernel - [PATCH v2 1/1] kmod: don't run async usermode helper as a child of kworker thread

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Message-ID: <20151015175239.GB28663@redhat.com>
Date:	Thu, 15 Oct 2015 19:52:39 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Frederic Weisbecker <fweisbec@...il.com>,
	Rik van Riel <riel@...hat.com>,
	Christoph Lameter <cl@...ux.com>, Tejun Heo <tj@...nel.org>,
	Rusty Russell <rusty@...tcorp.com.au>,
	linux-kernel@...r.kernel.org
Subject: [PATCH v2 1/1] kmod: don't run async usermode helper as a child of
	kworker thread

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
because auto-reaping only works if SIGCHLD is ignored, and 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@...hat.com>
---
 kernel/kmod.c | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/kernel/kmod.c b/kernel/kmod.c
index da98d05..0277d12 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, and we need a parent
+		 * that always ignores SIGCHLD to ensure auto-reaping.
+		 */
 		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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