[PATCH 1/5] cgroup: implement cftype->write()

Tejun Heo
2014-05-06T12:45:36Z

During the recent conversion to kernfs, cftype's seq_file operations
are updated so that they are directly mapped to kernfs operations and
thus can fully access the associated kernfs and cgroup contexts;
however, write path hasn't seen similar updates and none of the
existing write operations has access to, for example, the associated
kernfs_open_file.

Let's introduce a new operation cftype->write() which maps directly to
the kernfs write operation and has access to all the arguments and
contexts. This will replace ->write_string() and ->trigger() and ease
manipulation of kernfs active protection from cgroup file operations.

Two accessors - of_cft() and of_css() - are introduced to enable
accessing the associated cgroup context from cftype->write() which
only takes kernfs_open_file for the context information. The
accessors for seq_file operations - seq_cft() and seq_css() - are
rewritten to wrap the of_ accessors.

Signed-off-by: Tejun Heo <tj@kernel.org>
---
include/linux/cgroup.h | 27 +++++++++++++++++++++++----
kernel/cgroup.c | 14 ++++++++------
2 files changed, 31 insertions(+), 10 deletions(-)

diff --git a/include/linux/cgroup.h b/include/linux/cgroup.h
index 2dfabb3..da7a0ca 100644
--- a/include/linux/cgroup.h
+++ b/include/linux/cgroup.h
@@ -517,6 +517,15 @@ struct cftype {
*/
int (*trigger)(struct cgroup_subsys_state *css, unsigned int event);

+ /*
+ * write() is the generic write callback which maps directly to
+ * kernfs write operation and overrides all other operations.
+ * Maximum write size is determined by ->max_write_len. Use
+ * of_css/cft() to access the associated css and cft.
+ */
+ ssize_t (*write)(struct kernfs_open_file *of,
+ char *buf, size_t nbytes, loff_t off);
+
#ifdef CONFIG_DEBUG_LOCK_ALLOC
struct lock_class_key lockdep_key;
#endif
@@ -553,14 +562,24 @@ static inline ino_t cgroup_ino(struct cgroup *cgrp)
return 0;
}

-static inline struct cftype *seq_cft(struct seq_file *seq)
+/* cft/css accessors for cftype->write() operation */
+static inline struct cftype *of_cft(struct kernfs_open_file *of)
{
- struct kernfs_open_file *of = seq->private;
-
return of->kn->priv;
}

-struct cgroup_subsys_state *seq_css(struct seq_file *seq);
+struct cgroup_subsys_state *of_css(struct kernfs_open_file *of);
+
+/* cft/css accessors for cftype->seq_*() operations */
+static inline struct cftype *seq_cft(struct seq_file *seq)
+{
+ return of_cft(seq->private);
+}
+
+static inline struct cgroup_subsys_state *seq_css(struct seq_file *seq)
+{
+ return of_css(seq->private);
+}

/*
* Name / path handling functions. All are thin wrappers around the kernfs
diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index 57c2021..32d1624 100644
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -283,11 +283,10 @@ static inline bool cgroup_is_dead(const struct cgroup *cgrp)
return test_bit(CGRP_DEAD, &cgrp->flags);
}

-struct cgroup_subsys_state *seq_css(struct seq_file *seq)
+struct cgroup_subsys_state *of_css(struct kernfs_open_file *of)
{
- struct kernfs_open_file *of = seq->private;
struct cgroup *cgrp = of->kn->parent->priv;
- struct cftype *cft = seq_cft(seq);
+ struct cftype *cft = of_cft(of);

/*
* This is open and unprotected implementation of cgroup_css().
@@ -302,7 +301,7 @@ struct cgroup_subsys_state *seq_css(struct seq_file *seq)
else
return &cgrp->dummy_css;
}
-EXPORT_SYMBOL_GPL(seq_css);
+EXPORT_SYMBOL_GPL(of_css);

/**
* cgroup_is_descendant - test ancestry
@@ -1035,8 +1034,8 @@ static umode_t cgroup_file_mode(const struct cftype *cft)
if (cft->read_u64 || cft->read_s64 || cft->seq_show)
mode |= S_IRUGO;

- if (cft->write_u64 || cft->write_s64 || cft->write_string ||
- cft->trigger)
+ if (cft->write_u64 || cft->write_s64 || cft->write ||
+ cft->write_string || cft->trigger)
mode |= S_IWUSR;

return mode;
@@ -2724,6 +2723,9 @@ static ssize_t cgroup_file_write(struct kernfs_open_file *of, char *buf,
struct cgroup_subsys_state *css;
int ret;

+ if (cft->write)
+ return cft->write(of, buf, nbytes, off);
+
/*
* kernfs guarantees that a file isn't deleted with operations in
* flight, which means that the matching css is and stays alive and
--
1.9.0