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@xxxxxxxxxx>
Cc: Li Zefan <lizefan@xxxxxxxxxx>
---
include/linux/cgroup.h | 2 ++
kernel/cgroup.c | 37 +++++++++++++++++++++++++++++++++++++
2 files changed, 39 insertions(+)
--- a/include/linux/cgroup.h
+++ b/include/linux/cgroup.h
@@ -910,6 +910,8 @@ void css_task_iter_end(struct css_task_i
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);
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -277,6 +277,10 @@ static struct cgroup_subsys_state *cgrou
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 *cgrou
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)
{
--
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