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cgroup: introduce task_css_is_root()

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Determining the css of a task usually requires RCU read lock as that's
the only thing which keeps the returned css accessible till its
reference is acquired; however, testing whether a task belongs to the
root can be performed without dereferencing the returned css by
comparing the returned pointer against the root one in init_css_set[]
which never changes.

Implement task_css_is_root() which can be invoked in any context.
This will be used by the scheduled cgroup_freezer change.

v2: cgroup no longer supports modular controllers.  No need to export
    init_css_set.  Pointed out by Li.

Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Li Zefan <lizefan@huawei.com>
This commit is contained in:
Tejun Heo 2014-05-07 21:31:17 -04:00
commit 5024ae29cd
2 changed files with 16 additions and 1 deletions
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15
include/linux/cgroup.h
View file

@ -473,6 +473,7 @@ struct cftype {
}; };
extern struct cgroup_root cgrp_dfl_root; extern struct cgroup_root cgrp_dfl_root;
extern struct css_set init_css_set;
static inline bool cgroup_on_dfl(const struct cgroup *cgrp) static inline bool cgroup_on_dfl(const struct cgroup *cgrp)
{ {
@ -700,6 +701,20 @@ static inline struct cgroup_subsys_state *task_css(struct task_struct *task,
return task_css_check(task, subsys_id, false); return task_css_check(task, subsys_id, false);
} }
/**
* task_css_is_root - test whether a task belongs to the root css
* @task: the target task
* @subsys_id: the target subsystem ID
*
* Test whether @task belongs to the root css on the specified subsystem.
* May be invoked in any context.
*/
static inline bool task_css_is_root(struct task_struct *task, int subsys_id)
{
return task_css_check(task, subsys_id, true) ==
init_css_set.subsys[subsys_id];
}
static inline struct cgroup *task_cgroup(struct task_struct *task, static inline struct cgroup *task_cgroup(struct task_struct *task,
int subsys_id) int subsys_id)
{ {

2
kernel/cgroup.c
View file

@ -348,7 +348,7 @@ struct cgrp_cset_link {
* reference-counted, to improve performance when child cgroups * reference-counted, to improve performance when child cgroups
* haven't been created. * haven't been created.
*/ */
static struct css_set init_css_set = { struct css_set init_css_set = {
.refcount = ATOMIC_INIT(1), .refcount = ATOMIC_INIT(1),
.cgrp_links = LIST_HEAD_INIT(init_css_set.cgrp_links), .cgrp_links = LIST_HEAD_INIT(init_css_set.cgrp_links),
.tasks = LIST_HEAD_INIT(init_css_set.tasks), .tasks = LIST_HEAD_INIT(init_css_set.tasks),