From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754537Ab1LVBSk (ORCPT ); Wed, 21 Dec 2011 20:18:40 -0500 Received: from mail-ww0-f42.google.com ([74.125.82.42]:38453 "EHLO mail-ww0-f42.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752149Ab1LVBSi (ORCPT ); Wed, 21 Dec 2011 20:18:38 -0500 From: Frederic Weisbecker To: Li Zefan , Tejun Heo Cc: LKML , Frederic Weisbecker , Containers , Cgroups , KAMEZAWA Hiroyuki , Oleg Nesterov , Andrew Morton , Paul Menage , Mandeep Singh Baines Subject: [RFC PATCH] cgroup: Remove task_lock() from cgroup_post_fork() Date: Thu, 22 Dec 2011 02:18:31 +0100 Message-Id: <1324516711-26913-1-git-send-email-fweisbec@gmail.com> X-Mailer: git-send-email 1.7.5.4 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org cgroup_post_fork() is protected between threadgroup_change_begin() and threadgroup_change_end() against concurrent changes of the child's css_set in cgroup_task_migrate(). Also the child can't exit and call cgroup_exit() at this stage, this means it's css_set can't be changed with init_css_set concurrently. For these reasons, we don't need to hold task_lock() on the child because it's css_set can only remain stable in this place. Let's remove the lock there. NOTE: We could do something else: move threadgroup_change_end() before cgroup_post_fork() and keep the task_lock() which, combined with the css_set_lock, would be enough to synchronize against cgroup_task_migrate()'s change on child->cgroup and its cglist. Because outside that, the threadgroup lock doesn't appear to be needed on cgroup_post_fork(). Let's debate! Signed-off-by: Frederic Weisbecker Cc: Li Zefan Cc: Tejun Heo Cc: Containers Cc: Cgroups Cc: KAMEZAWA Hiroyuki Cc: Oleg Nesterov Cc: Andrew Morton Cc: Paul Menage Cc: Mandeep Singh Baines --- kernel/cgroup.c | 11 ++++++++--- 1 files changed, 8 insertions(+), 3 deletions(-) diff --git a/kernel/cgroup.c b/kernel/cgroup.c index 4936d88..d0dbf72 100644 --- a/kernel/cgroup.c +++ b/kernel/cgroup.c @@ -4622,10 +4622,15 @@ void cgroup_post_fork(struct task_struct *child) { if (use_task_css_set_links) { write_lock(&css_set_lock); - task_lock(child); - if (list_empty(&child->cg_list)) + if (list_empty(&child->cg_list)) { + /* + * It's safe to use child->cgroups without task_lock() + * here because we are protected through + * threadgroup_change_begin() against concurrent + * css_set change in cgroup_task_migrate() + */ list_add(&child->cg_list, &child->cgroups->tasks); - task_unlock(child); + } write_unlock(&css_set_lock); } } -- 1.7.5.4