From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752532Ab1LTTB1 (ORCPT ); Tue, 20 Dec 2011 14:01:27 -0500 Received: from mail-qy0-f174.google.com ([209.85.216.174]:33149 "EHLO mail-qy0-f174.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751625Ab1LTTBT (ORCPT ); Tue, 20 Dec 2011 14:01:19 -0500 From: Frederic Weisbecker To: Tejun Heo , Li Zefan Cc: LKML , Frederic Weisbecker , Containers , Cgroups , KAMEZAWA Hiroyuki , Oleg Nesterov , Andrew Morton , Paul Menage , Mandeep Singh Baines Subject: [PATCH 1/2] cgroup: Remove unnecessary task_lock before fetching css_set on migration Date: Tue, 20 Dec 2011 20:01:12 +0100 Message-Id: <1324407673-18975-1-git-send-email-fweisbec@gmail.com> X-Mailer: git-send-email 1.7.5.4 In-Reply-To: <1324320274-18485-1-git-send-email-msb@chromium.org> References: <1324320274-18485-1-git-send-email-msb@chromium.org> Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org When we fetch the css_set of the tasks on cgroup migration, we don't need anymore to synchronize against cgroup_exit() that could swap the old one with init_css_set. Now that we are using threadgroup_lock() during the migrations, we don't need to worry about it anymore. Signed-off-by: Frederic Weisbecker Cc: Tejun Heo Cc: Li Zefan Cc: Containers Cc: Cgroups Cc: KAMEZAWA Hiroyuki Cc: Oleg Nesterov Cc: Andrew Morton Cc: Paul Menage Cc: Mandeep Singh Baines --- kernel/cgroup.c | 21 +++++++++++---------- 1 files changed, 11 insertions(+), 10 deletions(-) diff --git a/kernel/cgroup.c b/kernel/cgroup.c index 5fdd98c..6b5f693 100644 --- a/kernel/cgroup.c +++ b/kernel/cgroup.c @@ -1850,13 +1850,13 @@ static int cgroup_task_migrate(struct cgroup *cgrp, struct cgroup *oldcgrp, struct css_set *newcg; /* - * get old css_set. we need to take task_lock and refcount it, because - * an exiting task can change its css_set to init_css_set and drop its - * old one without taking cgroup_mutex. + * get old css_set. We are synchronized through threadgroup_lock() + * against PF_EXITING setting such that we can't race against + * cgroup_exit() changing the css_set to init_css_set and dropping the + * old one. */ - task_lock(tsk); + WARN_ON_ONCE(tsk->flags & PF_EXITING); oldcg = tsk->cgroups; - task_unlock(tsk); /* locate or allocate a new css_set for this task. */ if (guarantee) { @@ -1875,9 +1875,7 @@ static int cgroup_task_migrate(struct cgroup *cgrp, struct cgroup *oldcgrp, return -ENOMEM; } - /* @tsk can't exit as its threadgroup is locked */ task_lock(tsk); - WARN_ON_ONCE(tsk->flags & PF_EXITING); rcu_assign_pointer(tsk->cgroups, newcg); task_unlock(tsk); @@ -2177,10 +2175,13 @@ int cgroup_attach_proc(struct cgroup *cgrp, struct task_struct *leader) /* nothing to do if this task is already in the cgroup */ if (tc->cgrp == cgrp) continue; - /* get old css_set pointer */ - task_lock(tc->task); + /* + * get old css_set pointer. threadgroup is locked so this is + * safe against concurrent cgroup_exit() changing this to + * init_css_set. + */ oldcg = tc->task->cgroups; - task_unlock(tc->task); + /* if we don't already have it in the list get a new one */ if (!css_set_fetched(cgrp, tc->task, oldcg, &newcg_list)) if (css_set_prefetch(cgrp, oldcg, &newcg_list)) -- 1.7.5.4