From mboxrd@z Thu Jan 1 00:00:00 1970 From: Li Zefan Subject: Re: [PATCH v2 cgroup/for-3.15] cgroup: make cgroup_enable_task_cg_lists() to grab siglock Date: Sat, 15 Feb 2014 10:11:47 +0800 Message-ID: <52FECCE3.1010707@huawei.com> References: <20140213182931.GB17608@htj.dyndns.org> <20140214204709.GB2851@mtj.dyndns.org> Mime-Version: 1.0 Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <20140214204709.GB2851-9pTldWuhBndy/B6EtB590w@public.gmane.org> Sender: cgroups-owner-u79uwXL29TY76Z2rM5mHXA@public.gmane.org List-ID: Content-Type: text/plain; charset="us-ascii" To: Tejun Heo Cc: cgroups-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, linux-kernel-u79uwXL29TY76Z2rM5mHXA@public.gmane.org On 2014/2/15 4:47, Tejun Heo wrote: > Currently, there's nothing explicitly preventing > cgroup_enable_task_cg_lists() from missing set PF_EXITING and race > against cgroup_exit(), and, depending on the timing, cgroup_exit() > seemingly may finish with the task still linked on css_set leading to > list corruption because cgroup_enable_task_cg_lists() can end up > linking it after list_empty(&tsk->cg_list) test in cgroup_exit(). > > This can't really happen because exit_mm() grabs and release > task_lock() between setting of PF_EXITING and cgroup_exit(), and > cgroup_enable_task_cg_lists() synchronizes against task_lock too; > however, this is fragile and more of a happy accident. Let's make the > synchronization explicit by making cgroup_enable_task_cg_lists() grab > siglock around PF_EXITING testing. > > This whole on-demand cg_list optimization is extremely fragile and has > ample possibility to lead to bugs which can cause things like > once-a-year oops during boot. I'm wondering whether the better > approach would be just adding "cgroup_disable=all" handling which > disables the whole cgroup rather than tempting fate with this dynamic > optimization craziness. > > v2: Li pointed out that the race condition can't actually happen due > to task_lock locking in exit_mm(). Updated the patch description > accordingly and dropped -stable cc. > I realise exit_mm() is a no-op for threads... There're quite a few places task_lock is used between exit_signal() and cgroup_exit(), but they're all conditional, so I think your original changelog stands!