Re: Racy manipulation of task_struct->flags in cgroups code causes hard to reproduce kernel panics

[Zefan Li <lizefan-hv44wF8Li93QT0dZR+AlfA@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.