cgroup: improve css_from_dir() into css_tryget_from_dir()

css_from_dir() returns the matching css (cgroup_subsys_state) given a
dentry and subsystem.  The function doesn't pin the css before
returning and requires the caller to be holding RCU read lock or
cgroup_mutex and handling pinning on the caller side.

Given that users of the function are likely to want to pin the
returned css (both existing users do) and that getting and putting
css's are very cheap, there's no reason for the interface to be tricky
like this.

Rename css_from_dir() to css_tryget_from_dir() and make it try to pin
the found css and return it only if pinning succeeded.  The callers
are updated so that they no longer do RCU locking and pinning around
the function and just use the returned css.

This will also ease converting cgroup to kernfs.

Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Michal Hocko <mhocko@suse.cz>
Acked-by: Li Zefan <lizefan@huawei.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Balbir Singh <bsingharora@gmail.com>
Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>

1 files changed, 16 insertions, 9 deletions

diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index 2de8decfd99f..fc2db071d95e 100644
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -4978,28 +4978,35 @@ static int __init cgroup_disable(char *str)
 __setup("cgroup_disable=", cgroup_disable);
 
 /**
- * css_from_dir - get corresponding css from the dentry of a cgroup dir
+ * css_tryget_from_dir - get corresponding css from the dentry of a cgroup dir
  * @dentry: directory dentry of interest
  * @ss: subsystem of interest
  *
- * Must be called under cgroup_mutex or RCU read lock.  The caller is
- * responsible for pinning the returned css if it needs to be accessed
- * outside the critical section.
+ * If @dentry is a directory for a cgroup which has @ss enabled on it, try
+ * to get the corresponding css and return it.  If such css doesn't exist
+ * or can't be pinned, an ERR_PTR value is returned.
  */
-struct cgroup_subsys_state *css_from_dir(struct dentry *dentry,
-                                         struct cgroup_subsys *ss)
+struct cgroup_subsys_state *css_tryget_from_dir(struct dentry *dentry,
+                                                struct cgroup_subsys *ss)
 {
         struct cgroup *cgrp;
-
-        cgroup_assert_mutex_or_rcu_locked();
+        struct cgroup_subsys_state *css;
 
         /* is @dentry a cgroup dir? */
         if (!dentry->d_inode ||
             dentry->d_inode->i_op != &cgroup_dir_inode_operations)
                 return ERR_PTR(-EBADF);
 
+        rcu_read_lock();
+
         cgrp = __d_cgrp(dentry);
-        return cgroup_css(cgrp, ss) ?: ERR_PTR(-ENOENT);
+        css = cgroup_css(cgrp, ss);
+
+        if (!css || !css_tryget(css))
+                css = ERR_PTR(-ENOENT);
+
+        rcu_read_unlock();
+        return css;
 }
 
 /**