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>

2 files changed, 37 insertions, 2 deletions

diff --git a/include/linux/cgroup-defs.h b/include/linux/cgroup-defs.h
index 10d814bcd487..df589a097539 100644
--- a/include/linux/cgroup-defs.h
+++ b/include/linux/cgroup-defs.h
@@ -84,6 +84,17 @@ enum {
 };
 
 /*
+ * cgroup_file is the handle for a file instance created in a cgroup which
+ * is used, for example, to generate file changed notifications.  This can
+ * be obtained by setting cftype->file_offset.
+ */
+struct cgroup_file {
+	/* do not access any fields from outside cgroup core */
+	struct list_head node;			/* anchored at css->files */
+	struct kernfs_node *kn;
+};
+
+/*
  * Per-subsystem/per-cgroup state maintained by the system.  This is the
  * fundamental structural building block that controllers deal with.
  *
@@ -123,6 +134,9 @@ struct cgroup_subsys_state {
 	 */
 	u64 serial_nr;
 
+	/* all cgroup_files associated with this css */
+	struct list_head files;
+
 	/* percpu_ref killing and RCU release */
 	struct rcu_head rcu_head;
 	struct work_struct destroy_work;
@@ -226,8 +240,8 @@ struct cgroup {
 	int populated_cnt;
 
 	struct kernfs_node *kn;		/* cgroup kernfs entry */
-	struct kernfs_node *procs_kn;	/* kn for "cgroup.procs" */
-	struct kernfs_node *events_kn;	/* kn for "cgroup.events" */
+	struct cgroup_file procs_file;	/* handle for "cgroup.procs" */
+	struct cgroup_file events_file;	/* handle for "cgroup.events" */
 
 	/*
 	 * The bitmask of subsystems enabled on the child cgroups.
@@ -336,6 +350,14 @@ struct cftype {
 	unsigned int flags;
 
 	/*
+	 * If non-zero, should contain the offset from the start of css to
+	 * a struct cgroup_file field.  cgroup will record the handle of
+	 * the created file into it.  The recorded handle can be used as
+	 * long as the containing css remains accessible.
+	 */
+	unsigned int file_offset;
+
+	/*
 	 * Fields used for internal bookkeeping.  Initialized automatically
 	 * during registration.
 	 */
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;