cgroup: generalize obtaining the handles of and notifying cgroup files

cgroup core handles creations and removals of cgroup interface files
as described by cftypes.  There are cases where the handle for a given
file instance is necessary, for example, to generate a file modified
event.  Currently, this is handled by explicitly matching the callback
method pointer and storing the file handle manually in
cgroup_add_file().  While this simple approach works for cgroup core
files, it can't for controller interface files.

This patch generalizes cgroup interface file handle handling.  struct
cgroup_file is defined and each cftype can optionally tell cgroup core
to store the file handle by setting ->file_offset.  A file handle
remains accessible as long as the containing css is accessible.

Both "cgroup.procs" and "cgroup.events" are converted to use the new
generic mechanism instead of hooking directly into cgroup_add_file().
Also, cgroup_file_notify() which takes a struct cgroup_file and
generates a file modified event on it is added and replaces explicit
kernfs_notify() invocations.

This generalizes cgroup file handle handling and allows controllers to
generate file modified notifications.

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Li Zefan <lizefan@huawei.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>

1 files changed, 13 insertions, 0 deletions

diff --git a/include/linux/cgroup.h b/include/linux/cgroup.h
index 355bf2ed8867..fb717f2cba5b 100644
--- a/include/linux/cgroup.h
+++ b/include/linux/cgroup.h
@@ -490,6 +490,19 @@ 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;