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cgroup: implement cgroup_get_e_css()

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Implement cgroup_get_e_css() which finds and gets the effective css
for the specified cgroup and subsystem combination.  This function
always returns a valid pinned css.  This will be used by cgroup
writeback support.

While at it, add comment to cgroup_e_css() to explain why that
function is different from cgroup_get_e_css() and has to test
cgrp->child_subsys_mask instead of cgroup_css(cgrp, ss).

Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Zefan Li <lizefan@huawei.com>
This commit is contained in:
Tejun Heo 2014-11-18 02:49:52 -05:00
parent 56c807ba4e
commit eeecbd1971
2 changed files with 39 additions and 0 deletions
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2
include/linux/cgroup.h
View file

@ -910,6 +910,8 @@ void css_task_iter_end(struct css_task_iter *it);
int cgroup_attach_task_all(struct task_struct *from, struct task_struct *);
int cgroup_transfer_tasks(struct cgroup *to, struct cgroup *from);
struct cgroup_subsys_state *cgroup_get_e_css(struct cgroup *cgroup,
struct cgroup_subsys *ss);
struct cgroup_subsys_state *css_tryget_online_from_dir(struct dentry *dentry,
struct cgroup_subsys *ss);

37
kernel/cgroup.c
View file

@ -277,6 +277,10 @@ static struct cgroup_subsys_state *cgroup_e_css(struct cgroup *cgrp,
if (!(cgrp->root->subsys_mask & (1 << ss->id)))
return NULL;
/*
* This function is used while updating css associations and thus
* can't test the csses directly. Use ->child_subsys_mask.
*/
while (cgroup_parent(cgrp) &&
!(cgroup_parent(cgrp)->child_subsys_mask & (1 << ss->id)))
cgrp = cgroup_parent(cgrp);
@ -284,6 +288,39 @@ static struct cgroup_subsys_state *cgroup_e_css(struct cgroup *cgrp,
return cgroup_css(cgrp, ss);
}
/**
* cgroup_get_e_css - get a cgroup's effective css for the specified subsystem
* @cgrp: the cgroup of interest
* @ss: the subsystem of interest
*
* Find and get the effective css of @cgrp for @ss. The effective css is
* defined as the matching css of the nearest ancestor including self which
* has @ss enabled. If @ss is not mounted on the hierarchy @cgrp is on,
* the root css is returned, so this function always returns a valid css.
* The returned css must be put using css_put().
*/
struct cgroup_subsys_state *cgroup_get_e_css(struct cgroup *cgrp,
struct cgroup_subsys *ss)
{
struct cgroup_subsys_state *css;
rcu_read_lock();
do {
css = cgroup_css(cgrp, ss);
if (css && css_tryget_online(css))
goto out_unlock;
cgrp = cgroup_parent(cgrp);
} while (cgrp);
css = init_css_set.subsys[ss->id];
css_get(css);
out_unlock:
rcu_read_unlock();
return css;
}
/* convenient tests for these bits */
static inline bool cgroup_is_dead(const struct cgroup *cgrp)
{