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Subject[PATCH v14 2/4] cgroup, block: implement task_get_css()
Date
From: Tejun Heo <tj@kernel.org>

Implement task_get_css() which find and pins the css for the specified
task and subsys. As a task is always associated with an online css
for every subsystem except while the css_set update is propagating,
task_get_css() retries till css_tryget_online() succeeds.

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Li Zefan <lizefan@huawei.com>
---
include/linux/cgroup.h | 25 +++++++++++++++++++++++++
1 file changed, 25 insertions(+)

diff --git a/include/linux/cgroup.h b/include/linux/cgroup.h
index 82319fb..a593e29 100644
--- a/include/linux/cgroup.h
+++ b/include/linux/cgroup.h
@@ -354,6 +354,31 @@ static inline struct cgroup_subsys_state *task_css(struct task_struct *task,
}

/**
+ * task_get_css - find and get the css for (task, subsys)
+ * @task: the target task
+ * @subsys_id: the target subsystem ID
+ *
+ * Find the css for the (@task, @subsys_id) combination, increment a
+ * reference on and return it. This function is guaranteed to return a
+ * valid css.
+ */
+static inline struct cgroup_subsys_state *
+task_get_css(struct task_struct *task, int subsys_id)
+{
+ struct cgroup_subsys_state *css;
+
+ rcu_read_lock();
+ while (true) {
+ css = task_css(task, subsys_id);
+ if (likely(css_tryget_online(css)))
+ break;
+ cpu_relax();
+ }
+ rcu_read_unlock();
+ return css;
+}
+
+/**
* task_css_is_root - test whether a task belongs to the root css
* @task: the target task
* @subsys_id: the target subsystem ID
--
2.4.2


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