cgroup: implement generic child / descendant walk macros
authorTejun Heo <tj@kernel.org>
Fri, 9 Nov 2012 17:12:29 +0000 (09:12 -0800)
committerTejun Heo <tj@kernel.org>
Fri, 9 Nov 2012 17:12:29 +0000 (09:12 -0800)
commit574bd9f7c7c1d32f52dea5488018a6ff79031e22
treef43657afb59dd12fe1eca329acc6caf885507727
parenteb6fd5040ee799009173daa49c3e7aa0362167c9
cgroup: implement generic child / descendant walk macros

Currently, cgroup doesn't provide any generic helper for walking a
given cgroup's children or descendants.  This patch adds the following
three macros.

* cgroup_for_each_child() - walk immediate children of a cgroup.

* cgroup_for_each_descendant_pre() - visit all descendants of a cgroup
  in pre-order tree traversal.

* cgroup_for_each_descendant_post() - visit all descendants of a
  cgroup in post-order tree traversal.

All three only require the user to hold RCU read lock during
traversal.  Verifying that each iterated cgroup is online is the
responsibility of the user.  When used with proper synchronization,
cgroup_for_each_descendant_pre() can be used to propagate state
updates to descendants in reliable way.  See comments for details.

v2: s/config/state/ in commit message and comments per Michal.  More
    documentation on synchronization rules.

Signed-off-by: Tejun Heo <tj@kernel.org>
Reviewed-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujisu.com>
Reviewed-by: Michal Hocko <mhocko@suse.cz>
Acked-by: Li Zefan <lizefan@huawei.com>
include/linux/cgroup.h
kernel/cgroup.c