From mboxrd@z Thu Jan 1 00:00:00 1970 From: Zefan Li Subject: Re: Racy manipulation of task_struct->flags in cgroups code causes hard to reproduce kernel panics Date: Mon, 22 Sep 2014 10:15:50 +0800 Message-ID: <541F8656.2030409@huawei.com> References: <201409192053.IHJ35462.JLOMOSOFFVtQFH@I-love.SAKURA.ne.jp> <541D16EA.70407@huawei.com> <20140920172819.GD3681@htj.dyndns.org> <201409211415.GJG26578.MFQOHtSFVJLOOF@I-love.SAKURA.ne.jp> Mime-Version: 1.0 Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <201409211415.GJG26578.MFQOHtSFVJLOOF-JPay3/Yim36HaxMnTkn67Xf5DAMn2ifp@public.gmane.org> Sender: cgroups-owner-u79uwXL29TY76Z2rM5mHXA@public.gmane.org List-ID: Content-Type: text/plain; charset="us-ascii" To: Tetsuo Handa Cc: tj-DgEjT+Ai2ygdnm+yROfE0A@public.gmane.org, miaox-BthXqXjhjHXQFUHtdCDX3A@public.gmane.org, peterz-wEGCiKHe2LqWVfeAwA7xHQ@public.gmane.org, mingo-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org, akpm-de/tnXTf+JLsfHDXvbKv3WD2FQJk+8+b@public.gmane.org, cgroups-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, linux-kernel-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, fernando_b1-Zyj7fXuS5i5L9jVzuh4AOg@public.gmane.org >> On Sat, Sep 20, 2014 at 01:55:54PM +0800, Zefan Li wrote: >>>> Then, what made current->flags to unexpectedly preserve PF_USED_MATH flag? >>>> The user is running cgrulesengd process in order to utilize cpuset cgroup. >>>> Thus, cpuset_update_task_spread_flag() is called when cgrulesengd process >>>> writes someone's pid to /cgroup/cpuset/$group/tasks interface. >>>> >>>> cpuset_update_task_spread_flag() is updating other thread's >>>> "struct task_struct"->flags without exclusion control or atomic >>>> operations! >>>> >>>> ---------- linux-2.6.32-358.23.2.el6/kernel/cpuset.c ---------- >>>> 300:/* >>>> 301: * update task's spread flag if cpuset's page/slab spread flag is set >>>> 302: * >>>> 303: * Called with callback_mutex/cgroup_mutex held >>>> 304: */ >>>> 305:static void cpuset_update_task_spread_flag(struct cpuset *cs, >>>> 306: struct task_struct *tsk) >>>> 307:{ >>>> 308: if (is_spread_page(cs)) >>>> 309: tsk->flags |= PF_SPREAD_PAGE; >>>> 310: else >>>> 311: tsk->flags &= ~PF_SPREAD_PAGE; >>>> 312: if (is_spread_slab(cs)) >>>> 313: tsk->flags |= PF_SPREAD_SLAB; >>>> 314: else >>>> 315: tsk->flags &= ~PF_SPREAD_SLAB; >>>> 316:} >>> >>> We should make the updating of this flag atomic. >> >> Ugh, why do we even implement that in cpuset. This should be handled >> by MPOL_INTERLEAVE. It feels like people have been using cpuset as >> the dumpsite that people used w/o thinking much. Going forward, let's >> please confine cpuset to collective cpu and memory affinity >> configuration. It really shouldn't be implementing novel features for >> scheduler or mm. >> >> Anyways, yeah, the patch looks correct to me. Can you please send a >> version w/ proper description and sob? >> > > This race condition exists since commit 950592f7b991 ("cpusets: update > tasks' page/slab spread flags in time") (i.e. Linux 2.6.31 and later) > but "struct task_struct"->atomic_flags was added in Linux 3.17. > > If use of ->atomic_flags for cpuset is acceptable, how should we fix > older kernels? Backport ->atomic_flags? > Yeah, we'll just add tsk->atomic_flags to struct task_struct when backporting this patch for stable trees.