cgroup: fix cftype->file_offset handling

6f60eade2433 ("cgroup: generalize obtaining the handles of and
notifying cgroup files") introduced cftype->file_offset so that the
handles for per-css file instances can be recorded.  These handles
then can be used, for example, to generate file modified
notifications.

Unfortunately, it made the wrong assumption that files are created
once for a given css and removed on its destruction.  Due to the
dependencies among subsystems, a css may be hidden from userland and
then later shown again.  This is implemented by removing and
re-creating the affected files, so the associated kernfs_node for a
given cgroup file may change over time.  This incorrect assumption led
to the corruption of css->files lists.

Reimplement cftype->file_offset handling so that cgroup_file->kn is
protected by a lock and updated as files are created and destroyed.
This also makes keeping them on per-cgroup list unnecessary.

Signed-off-by: Tejun Heo <tj@kernel.org>
Reported-by: James Sedgwick <jsedgwick@fb.com>
Fixes: 6f60eade2433 ("cgroup: generalize obtaining the handles of and notifying cgroup files")
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Acked-by: Zefan Li <lizefan@huawei.com>

1 files changed, 1 insertions, 13 deletions

diff --git a/include/linux/cgroup.h b/include/linux/cgroup.h
index 22e3754f89c5..f64083030ad5 100644
--- a/include/linux/cgroup.h
+++ b/include/linux/cgroup.h
@@ -88,6 +88,7 @@ int cgroup_transfer_tasks(struct cgroup *to, struct cgroup *from);
 int cgroup_add_dfl_cftypes(struct cgroup_subsys *ss, struct cftype *cfts);
 int cgroup_add_legacy_cftypes(struct cgroup_subsys *ss, struct cftype *cfts);
 int cgroup_rm_cftypes(struct cftype *cfts);
+void cgroup_file_notify(struct cgroup_file *cfile);
 
 char *task_cgroup_path(struct task_struct *task, char *buf, size_t buflen);
 int cgroupstats_build(struct cgroupstats *stats, struct dentry *dentry);
@@ -516,19 +517,6 @@ static inline void pr_cont_cgroup_path(struct cgroup *cgrp)
 	pr_cont_kernfs_path(cgrp->kn);
 }
 
-/**
- * cgroup_file_notify - generate a file modified event for a cgroup_file
- * @cfile: target cgroup_file
- *
- * @cfile must have been obtained by setting cftype->file_offset.
- */
-static inline void cgroup_file_notify(struct cgroup_file *cfile)
-{
-	/* might not have been created due to one of the CFTYPE selector flags */
-	if (cfile->kn)
-		kernfs_notify(cfile->kn);
-}
-
 #else /* !CONFIG_CGROUPS */
 
 struct cgroup_subsys_state;