cgroup: fix a subtle bug in descendant pre-order walk
authorTejun Heo <tj@kernel.org>
Fri, 24 May 2013 01:50:24 +0000 (10:50 +0900)
committerTejun Heo <tj@kernel.org>
Fri, 24 May 2013 01:50:24 +0000 (10:50 +0900)
commit7805d000db30a3787a4c969bab6ae4d8a5fd8ce6
tree5178e1adaa4f398d2c1367fb38633191e81764e5
parentd6cbf35dac8a3dadb9103379820c96d7c85df3d9
cgroup: fix a subtle bug in descendant pre-order walk

When cgroup_next_descendant_pre() initiates a walk, it checks whether
the subtree root doesn't have any children and if not returns NULL.
Later code assumes that the subtree isn't empty.  This is broken
because the subtree may become empty inbetween, which can lead to the
traversal escaping the subtree by walking to the sibling of the
subtree root.

There's no reason to have the early exit path.  Remove it along with
the later assumption that the subtree isn't empty.  This simplifies
the code a bit and fixes the subtle bug.

While at it, fix the comment of cgroup_for_each_descendant_pre() which
was incorrectly referring to ->css_offline() instead of
->css_online().

Signed-off-by: Tejun Heo <tj@kernel.org>
Reviewed-by: Michal Hocko <mhocko@suse.cz>
Cc: stable@vger.kernel.org
include/linux/cgroup.h
kernel/cgroup.c