Re: [PATCH v2 cgroup/for-3.11 1/3] cgroup: fix RCU accesses to task->cgroups

[Li Zefan <lizefan-hv44wF8Li93QT0dZR+AlfA@public.gmane.org>]

On 2013/6/26 2:48, Tejun Heo wrote:
> task->cgroups is a RCU pointer pointing to struct css_set.  A task
> switches to a different css_set on cgroup migration but a css_set
> doesn't change once created and its pointers to cgroup_subsys_states
> aren't RCU protected.
> 
> task_subsys_state[_check]() is the macro to acquire css given a task
> and subsys_id pair.  It RCU-dereferences task->cgroups->subsys[] not
> task->cgroups, so the RCU pointer task->cgroups ends up being
> dereferenced without read_barrier_depends() after it.  It's broken.
> 
> Fix it by introducing task_css_set[_check]() which does
> RCU-dereference on task->cgroups.  task_subsys_state[_check]() is
> reimplemented to directly dereference ->subsys[] of the css_set
> returned from task_css_set[_check]().
> 
> This removes some of sparse RCU warnings in cgroup.
> 
> v2: Fixed unbalanced parenthsis and there's no need to use
>     rcu_dereference_raw() when !CONFIG_PROVE_RCU.  Both spotted by Li.
> 
> Signed-off-by: Tejun Heo <tj-DgEjT+Ai2ygdnm+yROfE0A@public.gmane.org>
> Reported-by: Fengguang Wu <fengguang.wu-ral2JQCrhuEAvxtiuMwx3w@public.gmane.org>
> Cc: stable-u79uwXL29TY76Z2rM5mHXA@public.gmane.org

Acked-by: Li Zefan <lizefan-hv44wF8Li93QT0dZR+AlfA@public.gmane.org>