From mboxrd@z Thu Jan 1 00:00:00 1970 From: Mandeep Singh Baines Subject: Re: [PATCH 1/2] cgroup: Remove unnecessary task_lock before fetching css_set on migration Date: Tue, 20 Dec 2011 13:29:05 -0800 Message-ID: <20111220212905.GU13529@google.com> References: <1324320274-18485-1-git-send-email-msb@chromium.org> <1324407673-18975-1-git-send-email-fweisbec@gmail.com> Mime-Version: 1.0 Return-path: DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=beta; h=sender:date:from:to:cc:subject:message-id:references:mime-version :content-type:content-disposition:in-reply-to:x-operating-system :user-agent; bh=0oU7dtL0Ffg66jT8nP46u0bgQ/01bu5AnQJkztR4Vus=; b=BwytO7fwcWLJCvWNGN3xD9R+UKaz59yrBqBTkJDU2rPzWqsbkIzcDsq6zVKb/waxVO kYyn6gT8GAdrrPQ/jJMQ== Content-Disposition: inline In-Reply-To: <1324407673-18975-1-git-send-email-fweisbec-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org> Sender: cgroups-owner-u79uwXL29TY76Z2rM5mHXA@public.gmane.org List-ID: Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit To: Frederic Weisbecker Cc: Tejun Heo , Li Zefan , LKML , Containers , Cgroups , KAMEZAWA Hiroyuki , Oleg Nesterov , Andrew Morton , Paul Menage , Mandeep Singh Baines Frederic Weisbecker (fweisbec-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org) wrote: > 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 Reviewed-by: 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 >