cgroup: fix RCU accesses to task->cgroups
authorTejun Heo <tj@kernel.org>
Tue, 25 Jun 2013 18:48:32 +0000 (11:48 -0700)
committerGreg Kroah-Hartman <gregkh@linuxfoundation.org>
Mon, 22 Jul 2013 01:21:25 +0000 (18:21 -0700)
commitb6891ed4e66b65e5d6bb36964af0d65a08590018
treee076e3ab8dd110873fe714af8e27c2e6d395b617
parent83d0eb79752482bb888fb6d86ceed8971272f8b4
cgroup: fix RCU accesses to task->cgroups

commit 14611e51a57df10240817d8ada510842faf0ec51 upstream.

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@kernel.org>
Reported-by: Fengguang Wu <fengguang.wu@intel.com>
Acked-by: Li Zefan <lizefan@huawei.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
include/linux/cgroup.h