[PATCH v2 2/5] kernel: cgroup: add poll file operation

[Suren Baghdasaryan]

From: Johannes Weiner <han...@cmpxchg.org>

Cgroup has a standardized poll/notification mechanism for waking all
pollers on all fds when a filesystem node changes. To allow polling
for custom events, add a .poll callback that can override the default.

This is in preparation for pollable cgroup pressure files which have
per-fd trigger configurations.

Signed-off-by: Johannes Weiner <han...@cmpxchg.org>
Signed-off-by: Suren Baghdasaryan <sur...@google.com>
---
 include/linux/cgroup-defs.h |  4 ++++
 kernel/cgroup/cgroup.c      | 12 ++++++++++++
 2 files changed, 16 insertions(+)

diff --git a/include/linux/cgroup-defs.h b/include/linux/cgroup-defs.h
index 5e1694fe035b..6f9ea8601421 100644
--- a/include/linux/cgroup-defs.h
+++ b/include/linux/cgroup-defs.h
@@ -32,6 +32,7 @@ struct kernfs_node;
 struct kernfs_ops;
 struct kernfs_open_file;
 struct seq_file;
+struct poll_table_struct;
 
 #define MAX_CGROUP_TYPE_NAMELEN 32
 #define MAX_CGROUP_ROOT_NAMELEN 64
@@ -573,6 +574,9 @@ struct cftype {
        ssize_t (*write)(struct kernfs_open_file *of,
                         char *buf, size_t nbytes, loff_t off);
 
+       __poll_t (*poll)(struct kernfs_open_file *of,
+                        struct poll_table_struct *pt);
+
 #ifdef CONFIG_DEBUG_LOCK_ALLOC
        struct lock_class_key   lockdep_key;
 #endif
diff --git a/kernel/cgroup/cgroup.c b/kernel/cgroup/cgroup.c
index 7a8429f8e280..3f533f95acdc 100644
--- a/kernel/cgroup/cgroup.c
+++ b/kernel/cgroup/cgroup.c
@@ -3499,6 +3499,16 @@ static ssize_t cgroup_file_write(struct kernfs_open_file 
*of, char *buf,
        return ret ?: nbytes;
 }
 
+static __poll_t cgroup_file_poll(struct kernfs_open_file *of, poll_table *pt)
+{
+       struct cftype *cft = of->kn->priv;
+
+       if (cft->poll)
+               return cft->poll(of, pt);
+
+       return kernfs_generic_poll(of, pt);
+}
+
 static void *cgroup_seqfile_start(struct seq_file *seq, loff_t *ppos)
 {
        return seq_cft(seq)->seq_start(seq, ppos);
@@ -3537,6 +3547,7 @@ static struct kernfs_ops cgroup_kf_single_ops = {
        .open                   = cgroup_file_open,
        .release                = cgroup_file_release,
        .write                  = cgroup_file_write,
+       .poll                   = cgroup_file_poll,
        .seq_show               = cgroup_seqfile_show,
 };
 
@@ -3545,6 +3556,7 @@ static struct kernfs_ops cgroup_kf_ops = {
        .open                   = cgroup_file_open,
        .release                = cgroup_file_release,
        .write                  = cgroup_file_write,
+       .poll                   = cgroup_file_poll,
        .seq_start              = cgroup_seqfile_start,
        .seq_next               = cgroup_seqfile_next,
        .seq_stop               = cgroup_seqfile_stop,
-- 
2.20.1.97.g81188d93c3-goog